Having units swoop down and attack, or just fly above and be a terror unit are both impossible in this game. In the books the Fell Beasts didn't really attack at all, they were mostly just transport. A few attacked Faramir, but they didn't dive straight down to do it like eagles do. And one attacked Theoden and Eowyn from the ground. The Fell Beast is based off of a pterosaur, which was very capable of fighting on the ground. Just imagine a large velociraptor that can move just as fast on the ground and also fly!
So they, along with the bats, will just hover right above human sized units if everything looks okay, and I will make them fast and constantly charge.
The bats have a reason, I had no other rebel units to use for the Lindon area so it made sense. Otherwise I would not have included them.
Yes, their knees bend.