I imaged lotads to be something between fish and amphibian. Once they hatched and grown alittle, the lotad will locate an specific speices of water lily. The lotad will free the plants roots and then tangle up the plant around itself. The plant will make it harder for the lotad to swim, but will also provide the tiny pokémon with camoflage. When lotad evolves into lombre (lombre could be sort of amphibian), it will keep the lily entangled around it body. But when lombre evolves into ludicolo the plant will root and die as ludicolo is adapted to land life. The water lily get nothing out of this relationship so lotad can be seen as a parasite.
Something like that...