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-Paul Schafer (Pwyll Twiceborn) – Highly intelligent, withdrawn.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.
Jennifer is later revealed to be Guinevere reborn.
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 believe there is a six-legged horse, but some of the evil creatures, the urgach, ride six-legged beasts that are quite violent themselves. The elves have special horses of their own and there are only 5 of them left (and they no longer breed) which would put them as endangered.
As DavidW mentions, the 5 are not teenagers, but rather in their early to mid-twenties.
- Kevin is waiting to sit the bar after law school between books 1 & 2.
- Dave is a few years behind him in law school, borrowing his notes in the first book.
- Kim is in medical school and the Seer specifically mentions seeing her 25 years before. The following discussion shows that it was before she was born, but not by too much.
- Finally, Paul's girlfriend who died had just had her final graduate recital.