Slowbro?
The Shellder part of it is quick, has a shell harder than diamonds, and (being a clam-like pokemon) would produce a pearl. It hunts by biting a Slowpoke.
The Slowpoke part of it is slow and has a hard head (it can survive a bite on the head from Shellder and even benefit from it). It hunts by dipping its tail into the water to wait for something to bite.
The Shellder part of it is quick, has a shell harder than diamonds, and (being a clam-like pokemon) would produce a pearl. It hunts by biting a Slowpoke.
The Slowpoke part of it is slow and has a hard head (it can survive a bite on the head from Shellder and even benefit from it). It hunts by dipping its tail into the water to wait for something to bite.