The main thing to distinguish the animals you describe from man is the aspect ratio. Man is tall and thin and the other animals are shorter and wider (when viewed side on). Either you need to determine the height of the object your detecting so short objects detect and taller objects don't, or determine it's shape. An ideal job for computer vision and a poor job for something like an arduino. Maybe worth considering something like this http://www.aforgenet.com/framework/samples/computer_vision.html