Feebas in Gen III and IV. Just take a look at the chances of you finding the spots where you can fish for it. <_<
Actually, in Gen III if you actually managed to find a "Feebas square" he actually had a pretty high encounter rate. It was just a matter of actually finding that square... :/ --- I don't care if you hate every Pokemon Gen but the first one. Just don't get all up in my face about it and let me love all five.