Bronze Griffon
When animated, a bronze griffon acts in all ways like a normal griffon under the command of its possessor.
A normal griffon isn't trained as a mount (there's even a training fee specified in the MM). It could be commanded to accept a rider, though, but that wouldn't qualify him as "trained".
Also, remember you suffer some penalties on the Ride skill for riding bareback on a mount, so you should carry along a saddle if you plan on using this.