MoP brought us several animal bosses, currently too high level to tame. There's Oondasta and Thok the Bloodthirsty as Devilsaurs. There's Horridon the Direhorn.
And there's my favourite all-time desired beast: Tortos!
I would love it if there are any brave hunters in the Beta that can find out if these glorious beasts are becoming tameable! With special interest in Tortos, the most awesome of turtles!
(I know people want Megaera as Hydra, but his model is just heads, so not really suitable beyond imagination, cool as he would be.)
Edit: We have official word on this issue now. Blizzard tends to wait more than one expansion, before raid bosses are tameable.
Source: @Muffinus Twitter.