Deer. They're not the smartest creatures on God's green earth. Just look at how many you see dead on the side of the road if you need evidence of that. Any animal that can't look both way before crossing the road is definitely not intelligent. And in case you need more evidence, let the video below serve as that.
Watch as this poor 'lil deer attempts to mate with another deer. Unfortunately, the other deer is made of plastic and is a lawn ornament. Poor guy. But the woman recording the whole incident seems to be enjoying it!