This is an add-on which replaces the silverfish with a mouse. The mouse is naturally hostile towards players which it will chase after and bite if given the chance. If you want an easy way to defend yourself then get a domesticated cat. It’s actually a vanilla mob which is a part of the original game. Its behavior has been slightly modified to make it want to kill mice.
How does it work?
Domesticated cats (and ocelots) will chase after mice (actually they look more like rats) and try to kill them. You can domesticate a cat by feeding some raw fish to a youngling. You’ll have to stand perfectly still in order for them to approach you.
Here you can see the tamed cat chasing after a big and juicy mouse (which looks more like a rat than a mouse). The mouse is defenseless and will most likely die unless it manages to run away.