Well, the rulebooks didn't supply a sample picture, so I think you did okay. 
I think I read somewhere that it can move, while from the description you posted, I conclude it is rather sessile... Similarly to the "Kraken deal", I offer to do a different version if there are urgent requests.

Now, I guess we will just be waiting on me to do some mash-ups with Uatu's icons.
I think concerning the "lyccetaceans", that should not be much of a problem. Fortunately, Uatu did his wolves and his lion in different sizes...

I must admit I underestimated the effort concerning the rays: Uatu's Ixitxachitl looks like a small Manta Ray to me, while, if the D&D Stingray is identical to the real animal, it would look quite different and be much smaller.
Concerning the Pungi Ray, I found some data, but no info on size and no image. I guess from what I read that it is a ray with green spines on its back which can look like "sea weed". If I am right concerning the Stingray (normal size) and if you could give me some idea what the Pungi Ray looks like and how big it is, I could do both of them during the next days...