Sylvan Bloodline

At 1st level, you form a close bond with an animal companion. Unlike normal animals of its kind, an animal companion's Hit Dice, abilities, skills, and feats advance as the sorcerer advances in level. If a character receives an animal companion from more than one source, her effective levels stack for the purposes of determining the statistics and abilities of the companion. Most animal companions increase in size when their master reaches 4th or 7th level, depending on the companion.