The purpose of population is to limit the number of moving entities (units): the more there are, the greater the lag can become. War dogs cost 0 population, but have a hard limit of 20.
Domestic animals don't require population either and in principle it's possible for players to raise thousands of them. This patch adds an entity limit for animals to prevent that. Because an entity limit includes queued units (unlike population), it's set relatively high (50), to not make using corrals to get a food supply unviable.