What is the exact tradeoff versus a cannon? I suppose it's better than regular cannon balls, as a regular cannon ball just passes through one or two or three guys and that's it, whereas 50 arrows raining from above can hit a dozen guys or so. Sounds like its better than cannons until exploding shot starts showing up, at which point everyone in the vicinity is going to get shredded by shrapnel.
Armor. If the army are some kind of peasant rebellion, hitting a dozen or so guys may be worth it. Arrows are notoriously unreliable when it comes to incapacitating targets even then, but when the targets start wearing something like chain hauberk or halfplate, at long distance they are little more than an annoyance, even a decently sized shield with the right formation does wonders against arrows, ask Romans and everyone who used shieldwall tactics. Meanwhile a cannonball will kill or at least crush limbs even through the best of plate armor or shields.
Netting and barbed wire nets enclosing an installation would be a good deterrent for small-medium sized flyers like pegasi, I think.
Depends on how would they attack. Melee would be the riskiest option, i think the most popular nasty tactics any flyers would use would be to drop whatever incendiaries they can figure out. Worked great in WW2, even Korea and Vietnam, would work even better in times with even more construction being wooden.
Flying horses would be skeet, unless you armour them to an impractical degree. It's the same problem as with jetpack infantry. Up in the air, there's no cover.
"They're sending in their Winged Horses! Look, it's awesome!"
"Shut up and help me aim this thing!"
Nah, depends on the weapons available *A LOT*. There were 2 revolutions in AA gunnery's ability to hit targets. First one happened in mid-late WW2, with more complex sights, mounts and basic radar; the most obvious sign was turning the notoriously slow, low flying and vulnerable dive bombers from the scariest thing a soldier may see in the sky, which they were in early WW2, to the meme skeet and withdrawal later in WW2. Still, aircraft that did more evading and faster flying had a reasonable chance to not be hit, hence ground attack planes like Il-2 and A-2 Skyraider, nevermind fighter-bomber tactics like those used by USAF in Europe survived that revolution well enough.
The second revolution, from after which pic related comes, is fully radar guided, computer based automatic targeting, roughly late 70's technology . The weapon no longer relies on the ballistic skills and reflexes of the operator. He only picks the radar contact and decides he wants it gone, the computer does the predictive targeting, aims the gun at the right spot in the air, and picks the right time to shoot with precision much beyond what human reflexes, intuition and training allow. That's where the skeet meme comes from.
It depends how common they are but Pegasus knight’s could be used as scouts for sure, archers also, maybe bombers. But it would be pretty dumb/ risky to use them as shock cavalry.
Its all fun and games until the pegasus knights get revolvers and start fighting as Texas Ranger style cavalry.