posted by Strawberryhat .
Why is it called a string of ponies?
It has to do with the game of polo.
More than one pony per player is needed in order to allow tired mounts to be changed for fresh mounts between or even during chukkas. There are typically between 4 to 8 ponies per player. The group of ponies for a given player is commonly referred to as a "string of polo ponies", with a minimum of 2 or 3 ponies in Low Goal matches (with ponies being rested for at least a chukker before reuse), 4 or more ponies for Medium Goal matches (at least one per chukker), many more for the highest levels of competition.
Jillian's source seems to be this:
Please do not copy and paste directly from someone else's write-up unless you correctly use quotation marks and give the correct citation.
Simply stated it refers to the group of ponies of any given polo player.