Centaur spears can block many spells, but no one tries to block if they see that the spell is a certain shade of green. For this purpose it is useful to know some green stunning hexes.

At no point does Quirrell say "I just used such a spell on this centaur". I'm not ruling out that he killed the thing, and made an inferius in front of Harry. That would explain the unusually (?) sharp sense of doom that Harry felt when he "revived" it.

Also a possibility: memory charming a centaur is a lot harder, since they're only passingly similar to humans, so Quirrel had to draw more heavily on his magic, which in turn resulted in a sharper sense of doom.