Pseudis paradoxa is a hylid frog found across a surprising wide part of South America, and in Trinidad, but it wasn't the adult that captured everybody's attention, oh no. It was the tadpoles. And there were certainly eye-catching, being up to 25 centimetres long, just short of 10 inches. This, for example, is a small one.
Naturally, the natural historians promptly turned over the surrounding countryside trying to find whatever gigantic amphibian they grew into, and no doubt wondered how they could have missed something the size of a small dog. It was something of a shock, then, when they managed to grow some to maturity, that the adult turned out to be a quite ordinary-looking frog they been tripping over the whole time, one less than a quarter the size of the baby.
A wonderful 1800s engraving from John Good's Pantologia here, showing tadpole and adult together.
This is a development unique amongst amphibians, and most other species, for that matter, and we still don't know why they consider it necessary.