In mesopotamian mythology, Tiamat was at first the mother of the younger gods. When they had killed their father, Apsu, she made war on them, giving birth to monsters. After the younger gods killed her, they created the world from her body.
No, really. Not only did he mother Sleipnir, a horse with eight feet, after having mated with Svaðilfari, which we can reasonably characterise as a monster, we also have a short verse in Hyndluljóð, according to which he gave birth to "all the monsters" after eating the heart of a which (the verse is further discussed here).
For a more general discussion about the view of motherhood and monsters in Norse mythology, I would recommend Margaret Clunies Ross Prolonged Echoes, which devotes significant time to the theme.