Octopus.

I want one.

I mean, yes, I want a lot of animals, but I feel like an octopus would be especially fun to have despite the fact that I couldn't take it out of the water for fear it would go the way of the tiny water frogs of my youth (aka it would be dead and I would cry). But I could keep it in an awesomely cool tank and watch it wander in and out of glass bottles and hide underneath tons o' coral.

......and maybe poke at it occasionally.