What is a young frog called?

A tadpole, and it is a completely different animal from the adult - a filter-feeding or algae-scraping swimmer with a long coiled gut, gills, a tail and no legs. Turning into a frog means rebuilding almost everything: growing limbs, replacing gills with lungs, moving the eyes, rebuilding the skull, and shortening the gut drastically because a carnivore needs far less of it than a herbivore.

The tail is not shed. It is absorbed, digested from within by the tadpole's own enzymes, and the material is reused to build the new body - which is why a frog emerging from metamorphosis does not need to eat during the change. The whole process is driven by thyroid hormone, and a tadpole deprived of it simply never grows up.

Not every frog does it. Some species skip the tadpole entirely and hatch as miniature frogs, and there are stranger arrangements: the gastric-brooding frog of Australia swallowed its eggs and raised the young in its stomach, shutting off its own digestion to do it, and was studied with great interest as a possible route to treating ulcers before it went extinct in the 1980s. Amphibians are now the most threatened class of vertebrates on Earth, with a fungal disease spread by the pet and laboratory trade a large part of the reason.