Writers created Peewee and Tank for thier episodes, then they were just discarded since they served thier airtime. Hopefully we can get closure...or make a trip to the island of one episode only characters XD
The pets pretty much don't exist until the writers need them too... I imagine Spike & the mane six are horrible pet owners, half the time you don't even see the poor animals. Poor dears must be starving.
A thread has already been made about this. http://www.everypony.com/forums/showthread.php?4668-No-Love-for-Peewee