An early theme in several mythologies, particularly on the land masses referred to, in English, as Europe, Asia and North America, is that of women giving birth to giants, usually humanoid ones. In the chronology of the myths themselves, these women often live in the time prior to the Flood which covered the entire earth at some point thousands of years ago (and sometimes even millennia prior to the time of the peoples telling these stories, as far as they could reckon).
Disclaimer: Divine [and Semidivine] Women, Thus Technically Not Human
Pasiphae is one of a few female characters in Greek myth who are somewhat peculiar for the fact that they are actually immortal divine beings but who live among mortals as though they themselves are mortal, and they marry mortal kings, thus appearing to be human, even though they are not. Pasiphae's sister Kirke [Circe], a famous Odyssey character, is one such personage, who, according to one tradition, marries Telemakhos [Telemachus] son of Odysseus.
Pasiphae and Kirke are daughters of the sun-god Helios and the Oceanid Perseïs. In her book Gender and Immortality, Deborah Lyons notes the curious phenomenon in Greek mythology which has it so that some of the early female offspring of the gods (especially daughters of water deities like the rivers and the Oceanids) are immortal or at least semidivine (e.g. they become river-nymphs) while the early male offspring of these gods tend to be ordinary mortals.
Both Pasiphae and Kirke are born goddesses (albeit minor ones) while their brother Aietes [Aeetes] becomes the [evidently mortal] king of Colchis (where he becomes the keeper of the Golden Fleece before the Argonauts arrive in his country to claim it), even though they have the same mother and father. Aietes then goes on to marry his own aunt the Oceanid Eidyia. Of their children—a girl and a boy—the girl, Medeia, is sometimes described as having been born an immortal witch just like her aunts Pasiphae and Kirke, while the boy, Absyrtos, ends up dead just like any other mortal (having been slain by his own sister).
Before the Flood, a similar marriage like that of Aietes and Eidyia occurs between another Oceanid, Klymene, and King Merops of Ethiopia. Klymene marries Merops after having divorced Helios, who was married to her centuries before he was married to her sister Perseïs. Together, Helios and Klymene had had several daughters, all semidivine sun-nymphs, and a son Phaethon, who died taking his dad's car (the fiery Sun-chariot) out for a ride.
Yet another Oceanid, Pleïone, marries the Titan Atlas and bears him numerous daughters and a son. Seven of the daughters are called the Pleïades, after their mother. The eldest of them, Maia, becomes the mother of the god Hermes. Among the younger ones, Merope and Sterope marry the mortal kings Sisyphos and Oinomaos respectively. (Like his male cousins, the son here, named Hyas, dies a violent death, killed by a wild animal.)
All these [semi]divine women blur the distinction between the gods and humankind, bearing human children to their sometimes human, sometimes divine consorts, and often living as the unremarkable wives of their mortal husbands.
Pasiphae's (Older) Indian Equivalents
A similar phenomenon occurs in ancient Indian mythology, wherein the earliest women are the daughters of the ṛṣis [rishis], male entities who are a class of being distinct from the gods and men, and yet are related to both, and in fact, in most versions of the cosmogony, are the ancestors of almost all the other beings in the universe, including gods and humans. It thus becomes difficult to classify these daughters, who themselves marry other ṛṣis, to whom they are related (usually their own uncles or nephews or such), and to whom they bear a mix of children who vary in nature.
The foremost these are the many daughters of the ṛṣi Dakṣā [Daksha]. Depending on which source is consulted there are sixty or a hundred of them who then get married off to a number of other ṛṣis. Kāśyapa [Kashyapa], a relative of Dakṣā, married at least ten of these daughters and from them were born some of the most powerful gods and various tribes of āsuras [the traditional enemies of the gods], as well as humankind, plants and animals. Even though both Dakṣā and Kāśyapa are counted among the Prajāpatis, "Masters of Creation," entities who fashioned and organised the universe and its different lifeforms, they are sometimes depicted as ordinary human beings living on earth. This is especially the case with Kāśyapa and some of his wives.
Among the most notorious creatures classifiable as monsters in Indian mythology are the Rākṣasas [Rakshasas]. According to the Agni Purāṇa, the first of these were the thirty sons of Kāśyapa by his wife Muni, a daughter of Dakṣā. The Agni Purāṇa, however, was only composed sometime in the 600s AD.
There is an older record of other monstrous children of Kāśyapa, these being the first one hundred Dānavas, the first Daityas, and the first one thousand Kadraveyas. The mothers of all these children were daughters of Dakṣā. The Dānavas and the Daityas were closely associated groups of huge beings quite similar to the Titans of Greek mythology. Their mothers were Danu and Diti respectively. The Kadraveyas were a tribe of snake-people, several of them being many-headed creatures, whose mother was called Kadrū. One famous son of Kadrū is the gigantic snake-man Kāliya, who is described as having either 110 or 1,000 hooded cobra-heads. He was confronted and defeated once by the demigod hero Kṛṣṇa [Krishna].
The Dānavas, Daityas and Kadraveyas are mentioned, together with their parents, in the epic called the Mahābhārata. There are various dates posited for the composition of this literary work. On the later side we have the 900s BC—300s AD, although in Hindu tradition it was composed by one of the characters appearing in it, which characters are supposed to have lived potentially as long ago as the 5500s—3000s BC. This would make this, arguably, the oldest such record.
The earliest surviving mention of the Minotaur as a son of Pasiphae is Hesiod's Catalogue of Women, dated to as early as the 700s BC. However, if we cannot count Pasiphae or the daughters of Dakṣā as human beings, then we must look elsewhere for these human mothers of monsters. Hesiod again mentions certain giants whose mothers were human (assuming that giants count as monsters), namely Tityos [Tityus] and the Aloadai [or Aloadae or Aloads].
Tityos is said to have been a son of the earth-goddess Gaia but that is because his father Zeus, having seduced his real mother, the princess Elara, buried her alive in order to hide her from his jealous wife Hera. Therafter, from underground, Elara is supposed to have given birth to Tityos, who then emerged, in a double birth, from Gaia's womb: the ground.
The Aloadai were named from their stepfather Aloeus, but their real father was the sea-god Poseidon, who seduced Aloeus' wife Iphimedeia. From the union were born the twin giants Otos and Ephialtes, also known as the Aloadai, "Offspring of Aloeus." These two brothers were so massive and powerful that, while they were still children, they once captured the war-god Ares and stuck him in a jar, nearly starving him to death.
In his book The Mythology of the Wichita, describing the stories of an ethnic group from Oklahoma, USA, George Amos Dorsey recounts the story of how the chief or headman of a certain village had a wife who gave birth to four monsters. There are two versions of the narrative provided. The first says, ambiguously, that the monsters “had four feet”, which could mean that they each had only one foot or perhaps that each of them was a quadrupred who went about on all fours. The second version says that they were equine (horse-like), so perhaps quadrupedal. At any rate they began to consume the other children in the village and eventually the adults as well. They grew so huge that their heads touched the sky and they began to devour people from afar off, whom they were able to pick up by simply stretching themselves forth. In conjunction with a giant turtle, the Flood was sent to destroy them.
As this is based on oral tradition, it is difficult to know how old this story is, but it sounds very similar to events narrated in the First Book of Enoch, elaborating the beginning of Chapter 6 of the Bible's Book of Genesis. In the Hebrew text of Genesis, we are told that in the world’s early days benê ha-elohîm married and got children upon benoṯ ha-adam. These terms are variously translated and it is debated what exactly they mean. Benê ha-elohîm can be translated "Sons of God" or "sons of the gods," while benoṯ ha-adam literally means "daughters of [the] Adam" or "daughters of the Human" and is often rendered as "daughters of men."
The text also says that in the days that these marriages occurred there were the Nephilim on the earth or in the land. It is also debated what Nephilim actually means but in the Ancient Greek version of Genesis it is translated gigantes, which is typically rendered into English as "giants." 1 Enoch and the Book of Jubilees (which are considered to be apocryphal by most Jewish and Christian groups, although they are in the Biblical canon of the East African Tewahedo Churches) definitely follow this interpretation.
In 1 Enoch these giants are the children of a group of two hundred celestial beings, "angels," called the Watchers, who descend from the sky to the earth in order to consort with human women. Their monstrous offspring consume the beasts and the birds before becoming man-eaters and finally turning on one other. In response to their violence God deluges the entire earth to wipe them out.
Modern scholars date 1 Enoch to the 300s BC at the earliest. According to Tewahedo Church tradition, however, the book was actually written by Enoch, an early descendant of the first man Adam, and, according to the narrative, Enoch was born when Adam was still alive, which would make this text at least about as old as the Mahābhārata is claimed to be, if not more so.
(... and a few more giants)
If we cannot count humanoid giants as monsters then there are a few potential candidates attested starting around the turn of the 1st millennia BC & AD. In this time the Roman writers Ovid and Hyginus both make mention of Khrysomallos [Chrysomallus], the winged, talking ram whose golden fleece became the objective of the mission of the Argonauts about a generation after Khrysomallos’ death. Khrysomallos was the offspring of Poseidon [or rather Neptunus as he was known to the Romans] by a princess named Theophane. Poseidon/Neptunus had transformed Theophane into a ewe and himself into a ram while conceiving Khrysomallos. This ram occurs as early as the Catalogue of Women, but the most ancient surviving mentions of his origin seem to be in Ovid’s Metamorphoses and Hyginus’ Fabulae.
But perhaps a golden, winged, talking ram doesn’t sound very monstrous, in which case then the next timeframe in view might be the 100s AD, during which I can think of three options.
Erikhthonios [Erichthonius] was an early king of Athens and an ancestor of Theseus. He is very strangely described as either a dragon or a snake, or parts of his body sported the features of these reptilian creatures. Either his lower half was that of a snake or he had a snake’s tail with reptilian feet. Hyginus makes him sound pretty much like one of the Gigantes [Giants] who fought against the gods: the feet of these Gigantes were snakes. In order to hide his lower extremities, Hyginus says in his Poetica Astronomica, King Erikhthonios invented the four-horse chariot. The most common version of his parentage says that the king was the son of Gaia by the fire-god Hephaistos. According to Apollodorus, however, “Some say that this Erikhthonios was a son of Hephaistos and Atthis, daughter of Kranaos [Cranaus].” Kranaos was one of Erikhthonos’ predecessors on the throne of Athens.
The Seirenes [Sirens] are the famous part-bird monster sea-nymphs who feature as alluring songstresses in the Odyssey and in the story of the Argonauts. One of the Muses, either Terpsikhore or Melpomene, is usually said to be their mother, with the Aitolian river-god Akhelous [Achelous] as the father. Once again, however, Apollodorus records an alternate parentage in which the Aitolian princess Sterope, a daughter of Porthaon and Euryte and sister of King Oineus [Oeneus], is the mother.
Agrios and Oreios were a pair of man-eaters: grizzly bestial giants whose father was a bear. Their origin is quite similar to that of the Minotaur, except that their mother Polyphonte was much more human than Pasiphae. Polyphonte’s father was a man named Hipponoüs. Her mother Thrassa was the daughter of Ares by Teirene, a nymph of the Thracian River Strymon. The story of Agrios and Oreios is recorded by Antoninus Liberalis, whose work, entitled Metamorphoses, is thought to have been written sometime between 100 and 300 AD.