Is Treantmonk right or wrong about Mounted Combat?

[MaxWilson]

"Forest" implies a canopy and trees; these complicate both flying at near-ground-level and attacking from overhead.

Maybe in real life that would be true, but in the 5E ruleset, anywhere a Large Ettin can go at 40' speed, a Large Pegasus can go faster at 90' flying speed.

And it's not that the ettins get massacred if they leave their fortress. It's that they don't go picking a fight with a flying pegasus-rider out in the open. And they STILL can take steps to maneuver if he harasses them.

Frankly, "Bob on a Pegasus" should be using archery to attack them, which makes this less a problem of being mounted and more a problem of flight.

The burden of proof is on those complaining about "open field" simplifications to demonstrate that the simplification is losing a vital element. When the ettins are a mile away from their fortress, what specifically are they going to do that they couldn't do in an open field, and what terrain assumptions do they need in order to achieve it?