I Just Realized

Though tortoises are strong, their shell can only withstand so much weight before it breaks. As with other types of keratin, the load bearing capacity is high, but not high enough to support a full human body without cracking or separating.

Considering such information, would Tank really have been able to carry Rainbow Dash to the end of the race in "May the Best Pet Win"?

My answer may not shock you, but it's what I've come to believe. Pegasi are so light that their density is the same (or damn well near) the density of clouds. Their bones are light and thin, as are birds', and the feathers connect to the bone beneath the muscle at the front of their wings.

But if there are bones in the wings, why aren't there veins?

Let's think about that for a second. Birds have bones in their wings, but there are no visible veins like bats' wings. The answer lies in the fact that birds have a thin layer of flesh at the front of their wings, and this is where the bones lie. They don't extend to the back edge of the limb like with bats and other similar animals.

With pegasi, however, we have seen that the bones extend through to the ends of the feathers (Rainbow Dash's x-ray in S2E16). This is physically impossible given the design of pegasus wings, so I will have to discount it as an animation error.

And that is my (rather confusing) opinion on the design of the pegasus skeletal system. I'll discuss unicorns and earth ponies later if you guys want me to.

~SoI