He's a Pegasus on account of the wings he sports on the back of his vest, I added a faded white marking to them and his fetlocks to sort of simulate the look of the worn wing design on it. I was really pleased at the effect and have Lovey to thank since she picked the coat color. It makes a lot of sense to me, it's just like his signature winged leather vest.
There's a point during the episode in the second season 'Chupacabra' where Daryl's injured, arrow through his side and he's trying to climb up a very steep bit of land to get out of the wide wide creek bed. He glances down and mutters to himself something to the effect of, "C'mon, s'not that high, stop bein' such a pussy." then huffs and tries for the next pull up only to tumble all the way back down.
It leads me to think that maybe Daryl may have a small fear of heights. So the idea of him being a Pegasus is a little funny and potentially awesome as he dedicates his flight skills to darting around the woods. Crouching in trees and using his own feathers as his bolt's fletching.

If you need it:
His Cutie Mark feels right to me but it's still a bit of a mystery. It symbolizes his impeccable aim of course, his archery skills also since it's an archery target. His scars are on it. Some of them a complete miss, but the deepest are a bulls eye.
I think it has to do with how, when he'd be ignored or abandoned, like when he was a kid, it was to the extreme. But when he's focused on it's no less an extreme, either by getting shot by Andrea or utterly needed as the right hand man of the group leader.
As a pony, the Dixon brothers' Father scarring their Marks is quite heartbreaking since a Mark is supposed to be so very personal and treasured as a symbol of who you are.

Like I said. Still interpreting it as I'm discovering. But it does feel right.

Badassery and vulnerability abound in this pony interpretation. Two things I think really define the character of Daryl Dixon.