What needs to be done is to make a horse Fly, which is trivial enough. But then it also needs to be made to look like a Pegasus, including the wings flapping and folding.
Can Seeming spell do this?
Relevant snippets from spell description, emphasis mine:
The spell disguises physical appearance as well as clothing, armor, weapons, and equipment. You can make each creature seem 1 foot shorter or taller and appear thin, fat, or in between. You can’t change a target’s body type, so you must choose a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.
Clearly the wings can’t be new limbs, but could they be illusory magical wing items, made from real illusory Pegasus feathers? This would be same for the purpose of illusion, which doesm’t hold up to physical inspection anyway.
If Seeming can’t do this, what can, without requiring concentration?
I guess non-illusory fake Pegasus wings pilfered from an unlucky theater company could be an option, but I see complications in making them move while not being ripped off by wind…