equipment – What is the upkeep cost of a mount, animal companion, or familiar?

Additional creatures have the same upkeep as any player character

Companions, hirelings, familiars, and any other creatures traveling with the party should be accounted for when expending resources such as Rations and water. Paizo did not include any specific rules regarding larger creatures eating more, so it is up to the GM to determine if 1 week of rations is enough to feed your Large Boar for a week or only 3 days (as an example).

Most other maintenance is handwaved as part of the rules for Cost of Living and Subsisting. Other potential costs, such as stabling, are outlined in Services.

The bottom line is that, for an adventurer, the costs of maintaining allies should be minimal. Beyond the first level or two, unless your group has fun with micromanagement, it is unlikely to be relevant (except maybe for a long stretch of Downtime).