Lavar live on the bed of the ocean, usually in coral reefs. Most of their day is spent preening anything it comes across with it's many feelers. Humans have taken a fancy to their amazing ability to clean and have enlisted them as pets; taking advantage of their skills.
When a Lavar matures it becomes a Popolyp. During this stage of it's life it plants itself into the ground and becomes a stalk. It will grow many layers that form over each other many times. As it develops a sea snake will lay it's eggs on and around the Popolyp. The poisonous membrane of the Popolyp keeps predators away and the Popolyp eventually assimilates the eggs.
Around the time the eggs hatch the Popolyp goes through it's final stage of evolution to become the Medusa. The Medusa is controlled by and entirely dependent on the sea snakes. Without the snakes it would only drift aimlessly and slowly die of starvation.
There's me entry. I decided to stray a bit from the pokemon style because the only thing that doesn't evolve in pokemon games is the graphics. You'd think they could devote a bit more time to them considering they aren't animated and there are only two graphics per pokemon. (front/back)