Best I could find on a fast search (getting tired... old women need to sleep)
"The number of pets a character can own is a question that will likely be resolved through testing and other communication with the gaming community. It is likely, however, that a character will be restricted to one active pet at a time (given the detail level of our character models, too many would be a frame rate problem)."
-Melissa Meyer
"The relationship between a pet and an owner is far less casual than what is currently seen in MMOs. A player can define a unique appearance for his pet, and customize its abilities in order to create a pet that truly suits his character. How the pet grows will depend on the time, attention and personal investment that the player invests in the pet."
-Melissa Meyer
" The same goes for, say, a pet you may acquire. It can gain abilities and grow over time right along with your character."
-David Whatley, June 13, 2005