If you think about it, pegasi are some redundant animals. I mean, yeah, sure, you can run, and you can fly, but there are other things for those purposes.

You got actual horses for one thing, and you've also got those giant bird-things that you can summon with a moth from the Lord of The Rings

Even if they were able to to run and fly, the sheer muscle mass allowing a horse to be such a power-house when it comes to sprinting would be loss due to the fact that they'd need to shed extra muscle-mass off in order to take to the air, and they'd still be carrying a lot of weight, and possibly a rider, so their endurance and speed would be no good.

Also, horses should snack rather often due to their small stomach capacity. Ever notice how horses nibble on wild life when riding them? Yeah, and you're up in the air for hours at a time at a high altitude which is causing the air to thin, your pegasus is carrying you, and it is needing more oxygen to pump through its veins to make up for the thin air, causing it to wear-down even faster than it already would, and you need to heckin' feed it, but you also though it would be a good idea to go ride across the Atlantic for 23 hours.

You and your ride then spiral down into the depths of the sea.

you twit—

Oh, and coming across an airplane would then get you killed like a lamb to the slaughter which would then cause the engine to break which would then cause 100 people to crash and die.

bravo.