Aesir took Midgardsormur and threw it into the sea. It surrounds all land and still bites its tail. When the world will come to an end Midgardsormur will kill Thor.
Aesir took Hel and threw her into Niflheimur, up in the north. She is a queen of nine worlds where those who die of fever or old age go to after death. She can easily be recognized because half of her skin is blue but the other half with normal skin-colour.
Fenrisulfur was kept in Asgard. As Fenrisulfur got bigger and bigger Aesir had it chained but it'll brake away when the world comes to an end and it will kill Odinn.
When the Aesir punished Loki for causing Baldur's death they took his two sons, Vali and Narfi, and transformed Vali into a wolf. Vali then tore Narfi to pieces and the Aesir used Narfi's guts to tie Loki and put him deep down in the earth.
When Loki transformed himself into a mare he/it got pregnant with Sleipnir. Sleipnir has eight legs and runs faster than any other horse. It's now Odinn's horse and the best of horses. It can both run on earth and in the sky.