In Torchlight 2 there are something like 6 pets to choose from. Are these pets any different in ability or is it just an aesthetic difference?
Yes, as I came across. There will be times when you get gold or items dropping at a location where you or your pet on foot can't go.
A flying pet can easily go to that place Shift + Click to pick that for you.
Also, when you're on stairs, your ground pet will travel up and down stairs to kill monsters. Whereas a flying pet can fly over stairs (on aggressive mode) & clear the location for you.