Answering some questions will help get started on finding a pet you can afford:
What level are you? Do you like the idea of working for a long time hatching over and over to get the perfect pet? Or do you want/need something right now?
And answering the questions below will help narrow your choices down to the best pet for your playing style:
Do you need a pet for PVP or for the general game? Would you rather have mostly defense, mostly offense, or a mix of both? Or, would you rather have other stat boosts such as accuracy, healing, pip chance, etc. Are there any special talents/cards that you already know you want in a pet?
Knowing nothing about you and going off the top of my head, the best pets for Myth are the school exclusive pets for high stats & general talents; Crowns pets for moderately high stats & epic talents. You start earning school pets at level 48 and up. The better Crowns pets are White Tiger, Maple Moose, & the Cunning Seal Pup, but they're expensive.
There are also some really good dropped Myth pets, such as a Minotaur from Mooshu and a Myth Sprite you can win from Rattlebones. He drops a cute little one in Unicorn Way, or a much better one from Rattlebones Exalted & Archmage one-shot duels. There's also a Myth Treant you can buy for gold in the Wysteria pet shop.
Alia Misthaven
Emma Ravensong