Boston Low describes it as "A six-legged turtle-like creature". It has a light, mossy green skintone.
The turtle is seen on the museum spire, below the entrance to the museum. The Eel bursts out of the water beside the turtle and devours it, coming back later to spit out the skeleton of the unfortunate turtle.
This leads up to a jigsaw-style puzzle involving putting the turtle's skeleton back together, placing the explosive device into it, bringing it back to life with life crystals and allowing the eel to eat it. The eel then explodes, allowing you access to the underwater cavern.
The turtle puzzle is probably the most difficult puzzle in the game - you must correctly rearrange the bones according to the fossil you saw earlier.