While the shapeshifting Loki was technically Sleipnir's mother, he was father to Hel, the Midgard Serpent, and Fenrir. Their mother was the giantess Angrboða. According to part XII of "Gylfaginning," she lived in the Ironwood Forest, which was inhabited by troll women. Along with her three children with Loki, Angrboða was said to have many other sons who were giants that could take the shape of wolves. As noted in, "Norse Mythology: A Guide to Gods, Heroes, Rituals, and Beliefs" her name means "she who offers sorrow," because she was the mother of so many terrifying creatures.
As told in section XXXIV of "Gylfaginning" (hosted by Sacred Texts) the gods received a prophecy about the children of Loki and Angrboða would bring them "great misfortune... from the mother's blood, and yet worse from the father's." Odin, the king of the gods, believed the prophecy and had Loki's three children taken from Jötenheim. However, as noted in "D'Aulaires' Book of Norse Myths," he had sworn to be Loki's blood brother, and could not kill Loki's children even if they were destined to bring about the end of the world. For this reason, he sent Hel to rule over the dead, threw the Midgard Serpent into the ocean, and banished Fenrir to the island.
While the most famous of Loki's monstrous children were with Angrboða, he also had children with another woman – his wife.
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