When Typhon showed up at Olympus, most of the gods fled to Egypt in the form of animals. (Wikipedia is the only place I found what form they took.) Apollo was a hawk, Hermes an ibis, Ares a fish, Artemis a cat, Dionysus a goat, Heracles a fawn, Hephaestus an ox, and Leto a mouse. But I haven't found any sources on what animal Demeter transformed into.
Was it a mare? That would be my guess.
I can't find anything on Demeter at present. Wikipedia seems to be drawing from the Metamorphoses of Antoninus Liberalis, which has no mention of additional divine transformations:
Apollo became a hawk, Hermes and ibis, Ares became a fish, the lepidotus, Artemis a cat, Dionysus took the shape of a goat, Heracles a fawn, Hephaestus an ox, and Leto a shrew mouse. The rest of the gods each took on what transformations they could.
SOURCE: The Metamorphoses of Antoninus Liberalis; Typhon (trans. Francis Celoria)
Antoninus attributes this story to a lost work of Nicander's, Heteroeumena ("Metamorphoses"), but there is an account in Ovid, earlier than Antonius, which may draw partly from the same source:
Typhoeus hastened: but the Gods of Heaven deceptive shapes assumed. Lo, Jupiter, (As Libyan Ammon's crooked horns attest) was hidden in the leader of a flock; Apollo in a crow; Bacchus in a goat; Diana in a cat; Venus in a fish; Saturnian Juno in a snow-white cow; Cyllenian Hermes in an Ibis' wings.’
SOURCE: Metamorphosis (Ovid) 5.325 ff.
Graves offers an account identical to the Ovid:
When [Typhon] came rushing towards Olympus, the gods fled in terror to Egypt, where they disguised themselves as animals: Zeus becoming a ram; Apollo, a crow; Dionysus, a goat; Hera; a white cow; Artemis, a cat; Aphrodite, a fish; Ares, a boar; Hermes, and ibis, and so on.
SOURCE: The Greek Myths, 36.a
There may be alternate sources, but not all will be readily available online. So far, I've found nothing on Demeter transforming during this event, and it seems unlikely there is an account, but I'll continue to run down sources and see what I find!
There is a worthwhile note on this subject, drawn from Lucan [See note 3 to Apollodorus 1.6.3 (Frazer translation)]. Lucan's thesis is that these transformations were a device to explain the Egyptian worship of gods with animal forms. Presumably Hera becomes a cow because Hathor is seen as her analog. Demeter was heavily associated with Isis, who is not commonly associated with an animal form, and that may be a reason she is omitted.
From the birth myth of Arion, we are told (Pseudo-Apollodorus) that the horse was birthed by Demeter while she was in 'the likeness of a Fury'.
We are also told that Despoina is the daughter of Demeter and Horse-Poseidon.
Karl Kerenyi noted that she could take on the form of either an ear of grain, or a mare.
So it seems that your guess is right.
