I gave Shuckie back, without really meaning to. I went back to Cianwood to show the guy that I still had Shuckie, expecting that he would say I could keep it like he did in Gold. Instead, he took it back, which was a little disappointing.
I kept Kenya, though, just because I wanted to see if the sleepy guy on Route 31 would accept the mail if it was held by a different Pokemon. He did, and neither the guardkeeper or him would take Kenya back after, so I just put it back in my box, and never used it.