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?
This is probably not a very satisfying answer but it seems that his name isn't mentioned in any account of this story, and knowing that he comes from Jotunheim doesn't really help narrowing down the possibilities.
One thing to consider is the way the plowing of Denmark is related in the Ynglinga saga (which is the first chapter of the Heimskringla, which was also written by Snorri Sturluson):
Then he [Odin] sent Gefion across the sound to the north to discover new countries; and she came to King Gylve, who gave her a ploughgate of land. Then she went to Jotunheim, and bore four sons to a giant, and transformed them into a yoke of oxen.
Here it is explicitly said that she had those four sons precisely for the purpose of having them plough the land of Gylfi. From that, one can assume that which Giant fathered them didn't really matter to her, as long as it was a Giant, hence, maybe, why the Giant remained anonymous.
