[Uatu]

Hmm - well, I guess that the various armed men of the various nations were knights, of a sort - not quite Arthurian, but... And then we had those dark "bad knights," as well (they had armor, didn't they?)...

But yeah, I guess a Tolkein-based AD&D world would not really have cavaliers as a choosable class. I guess we could choose from:

Fighter
Ranger (Aragorn, etc.)
Magic-user/Illusionist (Gandalf, etc.)
Thief (Bilbo, etc.)

Of course, normal humans/elves/dwarves/hobbits would really only be able to choose from fighter or thief (mostly just fighter), really (seeing that you had to have special bloodlines or special origins to be the more magical classes).