Was thinking of mono-type Metapod here, but that'd be a bit ridiculous. Maybe for an uber-hard Scramble some time.
Instead, JukaPoke, you get a Slowpoke named Bro. Bro is, of course, very relaxed, and should therefore always be the slowest Pokémon in your party. To show that he is the bro-est of them all, Slowpoke can't evolve until it has defeated 5 other Slowpokes, while being slower too. Of course, it has to evolve into Slowbro. When it evolves into Slowbro, it must also defeat Will's (the E4 guy) Slowbro (while being slower, again).
In short:
- Get a Slowpoke, name it Bro
- Must be the slowest Pokémon in your party
- In order to evolve into a Slowbro, it must defeat 5 other Slowpokes while being slower
- Once a Slowbro, it must defeat Will's Slowbro, while being slower
Edited Will's Slowbro into this.