Isopods being a group of crusteceans found pretty much all over the place...many have probably seen the common pillbugs but there are plenty of species that are found underwater, same as most other crusteceans...Despite looking like a Trilobite/Horseshoecrab, Wimpods most likely based on these guys looking at it's segmented shell, antennae and tail...Purple colouring sort of matches also...I can actually imagine being able to encounter it either on land or water, given the terrestrial/aquatic nature of isopods (of course not much of a stretch since wild Shellos can be found on land)...also like Wimpod, they like to scavenge and can be quite skittish...
Edit: After some more digging Wimpod's most likely based on this guy...The Hawaiian Wharf Roach, which is of course an isopod...once again the colourations seem consistent as Wimpod has yellow/brown markings running down the center of it's back...
Of course some are the stuff of nightmares...but this guy's harmless though...a very suitable final form, perhaps with some tough armour making it a little tank...
One final thing They can be parasitic in nature sometimes as well...this is a form of symbiosis...Although it may be a bit uncomfortable at first the fish hardly notices once the isopod has fully settled in replacing it's tongue...The fish literally still uses it as it's original tongue...
Be an interesting outcome if Gamefreak does something with this...perhaps we could find a Wimpod using a Bruxish as a host or something...probably not, but still interesting to think about...