Have you come across a pony with this symbol on the bottom of one of her legs?

That special symbol designates that the pony that bears it is in the Dress-Up pose. Introduced in 2008, this symbol indicates a pony that can fit into special molded clothing and accessory pieces. The presence of this symbol means that this pony can freely trade these molded clothing pieces with any other similarly marked pony.

Most of the ponies released in 2008 were in a Dress-Up pose.

Dress-Up ponies, as well as their clothing pieces, are covered in greater detail in The My Little Pony 2007-2008 Collector’s Inventory.