To obtain a pet you have to get hold of a pet egg. These can be had by purchasing "huntsman traps" at the Sorrun family pet hunters in the cities. Only "newbie" zones have pet hunters - Logar, Varanas, Elven Island, Silvefall and Obsidian stronghold. There is also a pet hunter in Heffner camp in Coast of Opportunity.
Traps can then be used to catch magic cavies that spawn randomly from killed monsters. Once cavy is caught in a trap, you can get an egg from it. A huntsman trap might appear quite expensive at 8K gold each, but it pays off in the long run. Not only do you gain eggs that will grow your pet, but most of the eggs you can’t use can be sold for at least 8K in Auction House.
Alternatively, you can just buy an egg at Auction House. Good eggs are expensive, but since pets take a fair deal of money to raise and long time to mature, it doesn't pay to go with a substandard egg. By the time your pet is high level, initial pet cost will be relatively minor part of total cost. If you scrimp on the egg, you will sink lots of money into a substandard helper.
Catching a magic cavy requires a little skill. Once a cavy spawns you have to set up a trap by right-clicking it in your inventory. Then you need to make the cavy walk into the trap so that you can take an egg from it. To do so, you need to get behind the cavy in a way that cavy is between you and the trap. Cavy will run away from you, toward the trap, hopefully getting caught in it.
The sensor that makes it run from you seems to tick every few seconds so when walking around it to make it walk towards the trap stay far enough away from it so it won't go off in an unwanted direction. Once it gets close enough to the trap it will get caught.
You have one minute to complete the process before the trap expires and each cavy lasts 5 minutes. That means you often can use other people’s cavies if they are relatively fresh and people are not interested.
Sometimes on rare occasions cavy will run through trap and not get caught. If that happens, try to get on the other side of it and guide it back to the trap.
Cavies come in two varieties - "Magic Cavy" and "Golden Magic Cavy". The latter is very rare but tends to give you higher grade pet eggs.
-vfwiffo
http://forum.us.runesofmagic.com/showthread.php?t=53601
Video guide
https://www.youtube.com/watch?v=Hu9uae1QwR0 
