Orcs

[Knight of Gondor]

However, I think all discussion of elvish rebirth is pure speculation.

Actually, I read the concept of rebirth in either the Sil, or The Book of Lost Tales. I don't like the idea, because it is too similar to the New Age/Eastern religion and mystical ideas of reincarnation. But the book stated that Elves are reborn in their children somehow.

I do agree that each reproduces after its own kind, though, it sounds reasonable to me.