Mollux: The Lamp
It glows brightly when it is hungry. It lures prey with the light, then strikes quickly with acid.
Because its shell traps heat, it avoids the sun. Strange lights at night are often Mollux leaving the water.
Background info on my submission
Okay, I've found nothing in the rules that says I can't post this here, and I find it interesting how thinking of Dex entries changed my perception of this pokemon. First up is the species: Mollux is the "Lamp Pokemon" to satisfy my taste for simple, one-word classifications. I also find it appropriate that it shares its classification with Lampent.
The Black entry was literally the first thing that came to mind, as I'm sure it was for a number of people. I used acid as its hunting weapon because Mollux learns Acid at an early level, and doesn't learn Poison Sting at all. Sure, a reference to stinging can still work, since Dex-move inconsistencies happen sometimes with official pokemon, but I decided it wasn't something I wanted in my submission.
The White entry was trickier to put together. It was easy to think of the shell as an insulator and extend that thought to "Dry Skin = living underwater," but things got more complicated when I wanted to reconcile all that with a healthy ability to battle on land. In the end, I decided that rather than a pure water-dweller, Mollux should be amphibious, which I especially like because it provides more opportunity to use its Fire typing in the wild. While this didn't make it into my entries, I now picture Mollux using its poison to hunt underwater and its firepower on land (where it eats bugs I guess).
The routine of staying in water by day and emerging at night was inspired by, of all things, hippopotami. Not only does this allow for the reference to "strange lights" by the water, but I also enjoy the thought of how this could be implemented in an actual cartridge game. I'm picturing a land+water route where Mollux can be found by fishing during the day and in tall grass at night. It would be a fun flavour quirk IMO. The wording of the White entry ("Strange lights...are often...the water.") is a little odd and I'm not completely satisfied with it, but there's a precedent for odd writing like this in Gengar's entries ("The leer that floats in darkness belongs to a Gengar..."). I think this creates a good contrast between my entries: the Black entry uses a very straightforward, Pokemon-like style, while the White one is a little different but stays within the flavour of the games.