I've seen that pets are a common question around here. So I thought I would explain each one in as much detail as I can.
First of all, lets categorize them. There are gathering pets, skill pets, hunting pets, and transport pets.
The Gathering pets are Monkey and squirrel. Their only job is to pick up drops for the person, so they can just keep on killing. They are only available through the item mall. They must also be paid for once a month.
Skill pets belong in pacheon skill tree (bows). They are all birds. There's a white, black, and blue (in order of first-third book). The white and blue birds up attack rating. The black one attacks along with you. The black bird is the only one that falls under 2 categories of pets.
Hunting pets will attack monsters along with the owner. There are the wolf, and the black bird. Black bird is in pacheon skill tree. The wolves are available from two places, item mall and stable keeper. It costs 1million gold to get from stable keeper.
The wolf, you have to keep alive, it has 2 meters, one for hp, because it can get hit by mobs just as you do, and one for hunger points. As the wolf goes along, it gets hungry. If it nearly dies, remedy the problem with hgp pots from stable keeper. They say they heal 10 hgp, but that's misleading, they heal 10% of the hgp. To heal the hp, you use regular stable pots like you use for horses and transports. If your wolf dies, you can ressurect it with a grass of life from the stable. Be careful though, those cost 50,000 gold and can get expensive if it keeps dying.
Last, are transports. There are horses for transporting you from place to place at a faster speed. Then there are horses and camels you use for transporting goods from one town to another as a trader, or to thief town if you're a thief. As you level, your transport options get more health. In order to use the horse/camel transports, you must be wearing the thief or trader job suit. The travel transports can be used at any time starting from level 10. If any of your transports get hit by monsters or people, you can use the pots found at the stable to heal them back up.
This should help answer most questions dealing with most pets, if there are more, just ask and I will add them as I see fit to.
Quoted from BuDo
(Except I Am Vegeta cuz we all know he is a used tampon when it comes to his personality)