That's Guy Gavriel Kay's Fionavar Tapestry
From Wikipedia (with my emphasis):
The Five [travellers]:
Drawn by magic from our world and thus strangers to Fionavar, each of them finds a new role and a new destiny during their adventures in Fionavar.
-Kevin Laine (Liadon) – Witty, bright, outgoing. Fair of hair and of spirit. The act of love has a strangely deep and intense effect on him.
-Paul Schafer (Pwyll Twiceborn) – Highly intelligent, withdrawn. He is haunted by the death of his girlfriend in a car accident that he believes was his fault.
-Dave Martyniuk (Davor) – A large star basketball player, he carries emotional scars from his Eastern European immigrant father's rough treatment of him as a child and has difficulty accepting or offering friendship.
-Kimberly Ford (The Seer) – Quiet, intelligent and sensitive, but with the strength to make difficult choices and do what must be done.
-Jennifer Lowell – Green eyed, beautiful but cool and reserved. Former lover of Kevin's, still good friends.
From the summary of the first novel, The Summer Tree:
Paul is bound naked to the Tree where he hangs for three days and nights, fully expecting that he will die. On the second night, Galadan appears but is driven away by a grey dog. On the third night Dana, the Mother, relieves Paul's pain by showing him that he was not to blame for Rachel's death and Paul is at last able to weep for Rachel. His tears break the drought. Nursed (grudgingly) back to health by Jaelle, High Priestess of Dana, Paul recovers and is named Pwyll Twiceborn, Lord of the Summer Tree.
I don't remember a Sleipnir analogue, but there are a flying red unicorn and a Wild Hunt type group of riders in the later books.