well thanks for all

When you click on "Create a new character" it brings up the rules page and you must click "Continue" to make your character.
On that page, the last rule is;
"Sparkle Horses: Sparkle horses are not allowed on Spirit Source, as they were not seen in the Spirit movie, and none were in the books. All characters must be of natural coat colors/patterns and breeds (creatures such as unicorns weren't in the story!). Don't know about horse colors? Visit the Equusite or Wikipedia to find out, as well as to see pictures of each pattern. (Please be aware that some color genes are impossible in some breeds!)"

So no, a horse/eagle hybrid would not be allowed.
You can make a horse, or an eagle, but not a mix of them to make a pegasus type animal.

Technically, yes, Sparkle horses are indeed ALLOWED on Spirit Source. The mods just prefer you make them very integrated in the Spirit-Universe. If the character fits into the Spirit movie nicely, the mods will usually leave it alone. They only really 'attack' the poorly-made sparkle characters (like a pink horse who just happened to appear one day from nowhere.) The mod who made up the "No Sparkle Horses" rule just came up with that because they aren't fond on unrealistic horses. Sparkle characters are allowed on all of the sources as long as their bios are well-written.

Probably not. Spirit is a realistic film. Horses are OK, Eagles are OK, but Horse-Eagles? That's impossible. So it's un-realistic. But if you want creative character Ideas, think about other animals that may live around Spirit's area (Eg, Buffalo or Deer, maybe?).