Nor is the curvature right for anything else I can think of. Not a walrus, not a minotaur, not any horned animal we've seen so far. Out of all the possibilities, it seemed most likely to be a mammoth's tusk. I'll just assume it belongs to some new type of animal as hlvr suggested.
It's too small and the curvature is wrong. That's two strikes against it being a mammoth's with no points in it's favor. I'd go for a different or new creature, though.
And after looking at some pictures of cow horns, I found http://i.ehow.co.uk/images/a06/7k/bk/anatomy-cow-horns-800X800.jpg. That looks like a pretty similar shape to me. I think the problem is just the angle it's being viewed at.
Now compare it to http://www.joshuarue.com/mammoth_skull.jpg and it looks completely off.