most pokemon that are legends are based on mythical things not clearly able to be discerned at first glance. Most of the pokemon in 3rd gen like relacanth,wooper,mawile and castform and 4th gens like manaphy are based on things like this .
wooper is based on axolotol that never matures unless it undergoes a certain event (woah similar to special evoloution in pokemon)
mawiles is a yokai that has a head ( the mouth on its front)
castform is based on a water droplet that is designed after a small japanese charm that brings good weather
relicanth is based on a coaelecanth (name is misspelled)
phione and manaphy are cliones.
apparently zekrom is a gargouille and reshiram is a peluda.
btw they dont look like dialga or palkia albeit reshiram has the same claws in the feet as palkia and zekrom has dialgas feet