Maybe a Polar Bear, I personally think it should be Ice-Water type, or Grass-Ice. And maybe have the abillities: Swift Swim, or Thick Fat!
Polar Bear--->Large Bear
Sounds interesting at first... But I think it would be disappointing for it to evolve
out of being a Polar bear, personally. It's a pretty popular animal, so...
Maybe a Plankton pokemon. Plankton is a plant and is found under the sea, so a Grass-Water type, or Poison-Grass Type.
Plankton--->Seaweed--->Lily Flower
I think it would also be pretty disappointing to use Plankton as the plant rather than the
animal-like Plankton...
Besides, Lileep and Cradily are Sea Lilies...
I'd like some grass cats, although it would be odd to have ALL the cats as grass.
Maybe make one of them a Electric or something. I kinda thinking of how the Legendary beasts are, but more definiative of the species of animal that they serve
Like, a Fire-lion, or a Ground-ocelot or something.
Random idea... Make the ocelot a water-type, and they would probably be able to do something where they could make them "the supposed Pokemon that were re-incarnated into the Legendary Beasts". I can actually immagine Game Freak doing something like that. ...Maybe.
...Not saying I love the idea (kinda dumb, honestly), but it popped into my head and thought I'd say it anyway. *Shrug*
What would make a good typing for a flounder....*thinks*
Honestly, it would probably be biased into Water/Ground or maybe Water/Rock.
Though I could also maybe see a Water/Steel flounder, who's body is kind of like a "shield" on one or both sides. AMAZING defense, High special defense, good speed, though probably okay to moderately bad offenses...
Or maybe something like a Water/Ghost "Tattered Kite"-like flounder.