Page "Polo" Paragraph 38

Each player must have more than one pony, so tired mounts can be exchanged for fresh mounts between or even during periods, called chukkers, chuckers or chukkas.

A player's " string " of polo ponies may number 2 or 3 in Low Goal matches ( with ponies being rested for at least a chukker before reuse ), 4 or more for Medium Goal matches ( at least one per chukker ), and even more for the highest levels of competition.