A boner that you get when you see two ponies having sex. A boner that is slitely on the down side/that boner aftershock.
A few minutes after watching porn I had a pony boner. My two ponies stared having sex right I front of me and I got a pony boner so I had to get some tighter pants on. I got a pony boner when I sew the pony with his dick and balls hanging down.
An incidental erection. Usually common in the mornings, or with certain legwear.
My little brother was hanging around in his sweats yesterday and he had a fucking PONY BONER all goddamn day!