Gefjun, an Aesir, apparently had four ox sons with a giant:
But this woman was of the asa-race; her name was Gefjun. She took from the north, from Jotunheim, four oxen, which were the sons of a giant and her, and set them before the plow. Then went the plow so hard and deep that it tore up the land, and the oxen drew it westward into the sea, until it stood still in a sound.
Source: Gylfaginning (The Fooling Of Gylfe), Prose Edda by Snorri Sturluson, Wikisource
Do we know who this giant was?