Tiny, I don't know if it's really a Mustang thing or not, but I really noticed it more with my 2 than I have with any other horse I've ever ridden. Both of them had that attitude like that really upbeat waitress or cashier that makes you smile just because they're so happy.
Both of mine (and Dobe especially) just seem so happy to have a job and a life and a place to go. Leaving the barn or coming back, 2 minute ride or 10 hours, they've always got their ears perked and almost seem to ask with a smile "Okey-dokey, boss, what's next?".