Cocoons can be produced by many insects, such as the most common lepidopteran insects (silkworms, etc.). There is usually a pupa stage during the transition period from larvae to adults. This is what we often see. The silkworm chrysalis. Others, for example, the eggs of mantises also appear to be cocoons. I have seen a lot of silkworm pupae being sold in many feed stores for use as animal feed. They are actually quite clean. You can go and take a look.