If I remember it right, the pony has *very* high speed flaps.
Best way I've found to beat 'em is to wait for them to commit to the dive, dive yourself to high spee, then begin an oblique turn, watch him try to make it, and at the last second, pop the flaps to *really* haul it into a barrel roll.
Clean it up, go nose low, get some speed back- he'll try it again, and you'll do the same thing again. Eventually he'll tire of it or do something stupid.
If you get him to commit to a max G turn, you have him dead to rights.