Pets

Characters may invest in creatures from D&D’s various source books as pets and unusual servants. To determine the “purchase” value of a potential pet, look up the creature’s stat block and find the Challenge Rating (CR). The creature’s value is equal to the experience points it would earn you for beating it in combat … unless you’re proficient in the animal handling skill. In that case, you’re free to help train or domesticate pets as if you were making an artifice check (see Artifice for crafting guidelines).
How exactly the creatures come into your possession is at your discretion. Perhaps you captured it in the wilds and your expenses were whatever was necessary to contain and restrain it until you built up enough trust with it. Maybe this was actually the cost of buying and raising an egg. Maybe this was the cost of bartering the creature off a passing trader, or maybe it was the cost of the sealing magic that renders what would normally be an innately hostile monster docile in your care. Whatever the reason, the investment represents the effort necessary to obtain it and consider it yours to play with / as.
Once you own the creature, it is considered an unskilled Hireling in your service and thus becomes part of your upkeep costs. A creature’s upkeep cost is gold pieces equal to its CR (minimum 1), which covers food, shelter, and any other necessities to care for and keep your pet happy, healthy, and ho- … well, you get it. Occasionally, exceptions may be made (for example, a staff member may choose to reward you with a pet that supports you in combat), but these will be explicitly noted.
It should go without saying, but legendary or otherwise unique (named) creatures cannot be purchased under any circumstances.