The engravings are probably the work of giants, from this screenshot, I'm assuming that the giants tame them (Does this then mean that we'll see wild mammoths without the engravings? I guess we'll have to see.)
I never noticed them having four tusks until it was pointed out, though, interesting, I guess Bethesda wanted to distinguish them from real life mommoths, which is fine with me. I always knew there was no way Skyrim would be as exotic as Morrowind, but it's nice to see that Bethesda is still making an effort to make the setting feel more unique even though they want to make sure it fits the Psuedo-Viking theme of the Nords.
they look cool... if you don't count the bad textures. also look at their trunks, apart from their feet they're both in the same position completely, i hope this is a mere coincidence, we wouldn't want mammoths with inanimate trunks would we?
I'd imagine that the trunks are animated, however, it may be that this is their "default" position and the only move when performing certain animations, I guess we can't say for certain until we've seen them in action.