The design started with my idea that the frog monster could be transparent like this real-life amphibian:
And that it could also have a transparent cranium and eyes on the lower jaw. Here's some stills from the show:
I also contributed to the transformation of Ilana into a frog creature:
...as well as designing the frog guy's ship.
It's a tadpole.
Here's the episode- in spanish, lol: