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!
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.