The leg muscles - more specifically the quads. Only used when the legs are large/strong enough to merit their own mention. Hiking up one's pants or unisuit is another way of displaying ones ponies - this is referred to as a "pony show."
Used mainly by Long Island rowers or anyone similarly severely disattached from reality.
"Yo my ponies are yuge."
"We got an audience today, time to give em the pony show."
The state of mind one enters when one is so tired that positively everything, no matter how melancholy, serious or disturbing, is found to be utterly hysterical.
I haven't slept in three days; my dog just died, my car is on fire and I just watched ET, but I can't stop laughing, I clearly have the polyostrinchies!