Do you mean the animal in general, or what specific image? For Go specifically, it would be very surprising if it was not a gopher! Ok, Python is obviously even more closely associated with the snake, but a gopher has been the Go mascot from the very beginning (the original design being drawn by the wife of one of the language's co-creators)...

I meant in general.

For whatever reason, I forgot that Go uses a gopher as its mascot. But like, their PHP books don't use an elephant.

I dunno, I was just curious if the author could pick an animal to be on the cover, or if it wasn't their choice.

For Go it had to be a Gopher (as it is the mascot for the language). My editor showed me the artwork and I approved it.