Cloyster, The Bivalve Pokémon.
It fights by keeping its shell tightly shut for protection and by shooting spikes to repel foes. Once it slams its shell shut, it is impossible to open, even by those with superior strength. Cloyster that live in seas with harsh tidal currents grow large, sharp spikes on their shells.
Overview
Cloyster is a cool guy. Definitely one of my favourites (although some people think Shellder is cooler...). Back in 3rd gen is when it was really useful, being one of the few Pokémon to get Spikes and Rabid Spin, as well as being a strong physical wall. And then... Stealth Rock came into existence. Not only that, but everything got stronger moves such as Tyranitar upgrading to Stone Edge from Rock Slide, Fighting types upgrading from Brick Break to Close Combat, and pretty much everything being able to run mixed sets, which really destroyed Cloyster's usefulness as a spinner and a wall. So, this left Cloyster with being offensive as the only option, which with Skill Link it did quite well. However, when 5th gen was released, Cloyster got Shell Smash. Yeah, this one move skyrockets Cloyster's usability, and gives it the power to OHKO like... half of the OU tier. This overview is already huge so I won't go on about it but... +2 Attack, +2 Special Attack and +2 Speed at the cost of -1 Defence, which is stupidly huge so isn't a massive issue, and -2 Special Defence, which is stupidly low so isn't an issue either.