[Gandalf Parker]

Im jumping in late but a swallowing worm makes be think of two movies.
DUNE
TREMORS
maybe there is something out there on those that we can use

And since the images are mostly just for combat you might even get away with a dirt line for sprite one to simulate tunneling, and a head coming out of the ground biting for sprite two (combat sprite). Not the whole worm. Can it be given a big combat move to simulate the underground attack thing. Im not sure if flying has other factors you wouldnt want, such as not working in storms. But then again the water attacks use fly to simulate swimming so it might be the best available.