The selection Process in 5th gen and 6th Gen, it almost totally picked at random. Some Pokémon Starters, ledgendaries and a selection of special Pokémon are not on the random list. Furret was actually picked up during 3rd gen: http://serebii.net/potw/162.shtml (While it's not linked anyware on the site, you just need to know where to look or use the search option.
According to Smogon.. Furret is NeverUsed regular and I don't see it changing.. Personally I think it could be time consuming when using "competitive" Pokémon to complete against most NPCS.
Sorry, my edit didn't work. I did spot it in Gen III and tried to edit, but my phone doesn't approve....
And, well, I don't look at Smogon. It winds me up, but that's something else.