The term describes something that is exceptional, or awesome. It is derived from "pwn" which is ultimate nerd internet ownage. To own ponies is to do something amazing or be amazing.
The reverse of this, sucking, is to "blow ponies".
"She let me get to third base on the first date! Third basetotally owns ponies!"
A sexual act or photographic pose that involves the participant writing a large W on each butt cheek, and spreading them so that it spells the OwenWilson catchphrase "WoW"
She sent me some great pics last night, look at this OwenWilson