That is an African Dwarf Frog, and from the looks of it, it might be a male. Males have a white patch on their armpits after they reach 1/2 an inch. They also don't have a bump at where their tails would be, unlike females who have this bump.
When I first looked at the picture, I thought you had an ADF with a mutated head, then I looked closer and found out it was because of the curvature of the tank.
curious... umm.... so the guppy didn't have too much longer to go, leads me to think the guppy was sick.... would the guppy being sick affect the frog in any way? Can the disease be passed from fish to frog?
I was concerned about the guppy being ill myself. I was thinking about removing it from the tank not 2 minutes before the frog decided to eat it.
He was one of 6 male feeder guppies that I just purchased yesterday afternoon, to add some movement to the tank. Perhaps he was injured or stressed from the move rather than ill?