The first highly-descriptive, successful neuron model was designed from a squid axon (The Hodgkin Huxley model) because they have giant axons that those old electric probes could actually take meaningful measurements from.
We now use the Hodgkin Huxley model on mammals, too. In fact, I've used a genetic algorithm to tune a model designed for mollusks to match a rat neuron.
As far as morphology and form (looking at it under a microscope) I don't know. Given two animals of the same size, they few I have seen all look the same to me, but maybe there's some expert that knows a trick to tell the difference. Certainly, a squid axon is going to be much bigger than any human axon. But for two animal sof the same size, I don't think there's a lot of differences except for maybe in the case of different phylums. Even then, in my brief work on C. elegans, I didn't see much difference, but I'm a laymen as far as molecular biology is concerned.