This is a 'glass frog' (image from National Geographic):
It's one of a number of transparent or translucent creatures featured on the National Geographic's "Weird & Wild" blog. (Actually I take issue with the Monarch butterfly image there, as strictly speaking we're seeing a transparent pupal case; the butterfly inside is definitely not see-through.)
Glass frogs (Hyalinobatrachium pellucidum) are on the ICUN's 'red' list as an endangered species, with habitat destruction the likely cause. However, if chitrid fungi are introduced to the frog's limited range – they're recorded from only five locations on the Amazonian slopes of the Andes in Ecuador – then the population will likely decline even faster (always supposing this particular pathogen isn't already there). These delightful little frogs are apparently about the size of a fingernail, & their translucency is due to a lack of pigment in the skin. Not only can you see the air-filled lungs, the red threads that are blood vessels, and the heart with some of the major arterial arches clearly visible – you can also see the animal's skeleton.
And that reminds me: we were talking in class the other day about gastric-brooding frogs & one of the students said they'd heard that this species had been cloned. An intriguing possibility – I must go off & look into it!