I have tried the following to no success;
Changed the ID to increment, either caused a blank spawn egg that did nothing or still crashed the game when the first entity was spawned
Tried changing the model, encase there was an error there, still crashed when one was spawned
Tried redoing the texture to a solid colour encase there was an issue with the texture, still crashed
All of these with the same error as before, line 24 in the render file where it returns the texture for the entity.
Any other advice or things to try would be nice, or even a nudge in the direction of a tutorial guide my students could follow that someone knows to work, or, any files that anyone has where they have created multiple custom mobs would also be great
Thanks for the help so far