Like 2horses said, pretty much anything 14.2 & under that appears stock type can be registered as a quarter pony.
That being said, depends on who you talk to what they may consider a QP. For me, a quarter pony is a short quarter horse, anything else is a pony cross (unless it's a registered breed like Welsh, etc). But that's just my personal take because it makes it more clear what you may/may not be purchasing. If it's a QH or if it's a cross breed, I want to know what one truly is not just a slapped on label because of size. Here's our resident QP gelding, Stubby, 13.2hh AQHA reg'd son of Reynolds Rap (son of Sonny Dee Bar). Just don't tell him he's a pony, he'd take offense.
Quite some years ago with my sister on board IMAG0610.jpg
A few months back in all his fluffy winter glory with my DD. Stub recently celebrated his 21st birthday. IMAG1042.jpg