I think that first image just solved a mystery for me.
In the
unpointed Prerelease text there's a part that takes place in prehistory in block 5 that briefly mentions a character called "Migo" (ミゴー). He seems to surprise the party somehow, and Lucca reacts by saying something like "What the, It's only Migo..."
Then there's the unused triceratops graphics from the Prerelease. This guy:
I think that's Migo. The big triceratops in the background of the first sketch you posted is labeled "ミゴー", just like the character mentioned in the unpointed text. Rather than an unused enemy, I think he was a friendly NPC. Like Ayla's pet or something. Explains the happy-faced tail wagging animation (the animation data seems to be gone, but the sprite assembly data is still there, and I used it to make this image. The palette is Nizbel's. Only this palette and the Frog King palette really seemed to have the color ramps in the right place, and the Frog King palette makes him look borderline radioactive.)
Edit: Just for funsies,
here's a gif of Migo puttering around the house.