Umm this does look interesting. I also know a few commercial adoptables/pets script, one of them even has a simple battle system. Unfortunately, none of these is still under development. Apparently adoptables script itself is not profitable enough for any company to make money, it is better for true adoptables lovers to develop scripts.
And incase you wonder, that script's last update time was July 2009, before I got to know BMR and Rusnak Adoptables:
http://kitto.yasashiisyndicate.org/downloads.html