Crud

You can find only 1 crud in an entire generated minecraft world, so it may take years to spot one even with active searching.

In the game data crud is labelled as Earwig, but was published in an unofficial minecraft guidebook under the name crud, which became more popular among the crud hunting community

Crud moves very slowly and is the size of a silverfish, making it easy to miss even if you do pass one by. If it is killed or spawns in an unfortunate place and dies, it does not respawn

The only way to interact with crud is to feed it a coarse dirt block, which causes an achievement window to open with the word "null". After when checking your recorded achievements, it is not listed anywhere

A seed with crud close to spawn has yet to be discovered, and summoning one manually is not possible without complicated hacking and modification to the game