We don't seem to have a graphics preview yet, but I'm watching the Inside Howrse blog for a preview of the character models; I'll post when they come out. If they come out, that is.
Howrse says they'll breed like Donkeys- only 3 times in their life. Other things- comps, specialties, boarding, BMIs, BLUPing and skilling and GP and Excellency stars and etc will all be just like normal horses.
Missions, on the other hand- will be totally doubled. Double skill gain, double the money for the owner, double the resources for the EC, etc.
Howrse also added that "creations are accepted" but I don;t know if they mean they are CURRENTLY being accepted or are going to be once the breed is added. If a Black Pearl user could let me know in the comments, that would be awesome!
More information will come as soon as I know- check back!
As always, happy Howrseing,