He told pidgeot he'd be back when he was back from delivering the GS ball.
He told Butterfree he'd tell the other pokemon he was just on a vacation, and would come back one day.
Lapras did come back for one episode.
Larvitar... He had a close bond with him, but still returned him to his mother... I could see Ash running into him in this seasons "Battle frontier" involving the fight, resort, etc, areas. As one IS a desert...
Primeape he gave to someone to train, in the original japanese version, I don't believe he ever said he'd come back, but I could be wrong...