It's a bit farfetched, but shellder seem to evolve when they
latch onto slowpokes tail (in the anime appearance) I was
thinking that slowpokes tail has energy similar to a water stone
in it and when a shellder latches onto it's tail it evolves
except it's not in it's shell. Also the "shellder" on slowbro's
tail has the same evil smile. On the other hand it could be that
slowbro "shellder" is a theoretically a branch evolution of
shellder except you can't use it it's just on slowbro's tail.
What do you think?