Weezing and Arbok have been released to guard a group of Ekans and Koffing.
Butterfree was released also, to accompany a Shiny Butterfree.
Ash's Pidgeot was left to guard a flock of Pidgey and Pidgeotto, and Ash did say he would come back for it. Possibility.
Ash's Primeape was left in training. Remember that Charizard was left to train as well, and eventually came back. Perhaps this will be the same with Primeape, but not very likely.
No exact information can be given, because these haven't happened yet.