1. There are two kinds of sloth, the Two-toed Sloth and the Three-toed Sloth. The Two-toed sloth, rather confusingly, has three toes (but two fingers). The two kinds pretty much look the same, do the same things and live in the same places.
2. The sloth is the only animal in the world to not have seven cervical (or neck) vertebrae (apart from manatees). BUT the Two-toed and Three-toed sloths have a DIFFERENT number of vertebrae. The Two-toed has six and the Three-toed has NINE.
3. This crazy vertebrae inconsistency is because the Two- and Three-toed sloths do not have a shared ancestor until you go back 40 MILLION YEARS. They are a brilliant example of convergent evolution, where critters look the same and do the same things, because they've been surviving next to each other in the same environment for a very long time.