From the article, "All pets in Mabinogi cost real money. This means you can't just pick one without at least a little thought. After all, you don't want to spend $5 on a pet that isn't going to compliment and assist your character.
Your best bet is to learn about the best parts of all the pets available so you can make an informed decision before adopting.
I'd suggest going with two pets for your primary character and one for any secondary characters you may play as. For the two primary pets, go for those that compliment each other. Perhaps choose one pet that boosts critical hits or one who boosts strength. Then, for the secondary characters' pets, choose pets that directly affect those characters' strengths..."