Dolphin

Or perhaps a shark? Seeing as it only steals your fish and doesn't cause you to lose a life at all, I assume it's a bit friendlier than a shark!

Graphics_GoldfishGameDolphin
Marlin

Although there are four frames, they are all the same data.

Graphics_GoldfishGameMarlin
Sea Snake

I guess on reflection, it could be an eel!

Graphics_GoldfishGameSeaSnake
Squid
Graphics_GoldfishGameSquid