Perfect(ly useless) Pokemon

Metapod evolves into Butterfree, and Butterfree has access to Bug powers, and Mewtwo is weak to Bug. So you now have a 100% counter to Mewtwo!

...

you didn't specify that it had to be a GOOD reason...

I caught a level 28 Murkrow yesterday 15a/15d/0s...double dark. Probably keep it, I've only held on to one other Murkrow, an 82% level 30

Level 1 15/15/15 Fearow, one of my greatest trophies

I caught a level 30 100% pidgey yesterday. Just going to keep it, only 1 in 120000 wild catches will be level 30 and 100%. The mons don't take up that much space

You never know when the game will change in order to make more mons useful. One can only hope, I've been holding on to lower tier mons with high IVs just in case they become useful again.
(of course a few months ago I completely cleaned out my bag of "useless" mons. I go in cycles....Clean out, start over, clean out, start over. I'm in my start over cycle now for those types of mons)

Congrats on your perfect metapod!
I have kept and evolved all of my perfects (Arcanine, Pidgeot, Blastoise, Ninetales, Lapras, Poliwhirl). I went 6 months without getting one last time so I will never toss one no matter how bad a species it may be. They are some of my favorites, I maxed out the Arcanine and gave the Ninetales a Fast TM to get it double fire moves. I'm powering up the Lapras and saving the Blastoise for far down the road with megaevolutions and I'm planning to evolve the Poliwhirl to a Poliwrath and get it double fight moves.

No, useless is by definition not useful.

is a shiny usefull? No. But still everyone wants them. I have a Sandslash, this cow thing and a Raichu. I never used them and obviously never will. But I'll keep them for sure. I'll also keep my 0% Magikarp.

I'd keep it and evolve it, but I am a hoarder and I collect everything rare or exotic I find, regardless of its usability. My very first perfect pokemon was also my first Lapras (hatched). The one and only Moltres I could catch was also a perfect one (luckiest valor player in the world). I am also the proud owner of a perfect Noctowl, a perfect Espeon, a perfect Aerodactyl, a perfect Sandslash, a perfect Charizard and a perfect Dodrio, all wild catches. Oh, and a perfect Dragonite, also hatched and only the fourth I've ever gotten.