They're a bit bigger with a horn in the middle of their foreheads, but otherwise it's not a dramatic outward difference. The rapidash could have easily blended in with a herd of ponyta.

[Particularly from a distance, and if they're walking around or grazing.]

The biggest difference is in their speed. I don't think I've ever encountered a faster pokemon. I suspect when the conflict between Gigas and Caldera is resolved that rapidash racing is going to become a trend in Union.