A big green horn worm. They eat plants and destroy tomato crops. So they should be killed if one is seen. Although otherwise harmless.
I think that it is odd sometimes that I have been alive this long and yet never noticed this worm. Actually it is the perfect shade of bright yellow green that blends into the grass and goes unnoticed. Natures camofloge at work. Once you are aware though you will see them everywhere. The horn on the tail is just a decoration I was informed.
Grant in his Grandpa Dawkins overalls quite a handsome fellow if I do say so and I do!