Still talking about Airs Above The Ground so they're all together.
The levade is one of the most difficult things you can ask a horse to do, physically.
Unlike the pasade, where the horse rears at the natural angle for a horse rearing, the perfect angle for the levade is 45 degrees. So, why is it so hard?
Balancing in the levade requires amazing core strength on the part of the horse. Even the Spanish Riding School only has select horses do it - and some horses can never manage to learn it.
Look at this picture of Neapolitano Aga at the Spanish Riding School (source: David Monniaux via Wikimedia Commons).
Notice the quite visible horizontal "line" behind the saddle that curves upwards as it approaches the groin.
That's the horse's abs. You don't see that kind of development on most riding horses.