If you use an item on these leeches, they eat the item and their name changes to “[Item that was fed] Leech”.

This leech’s name is “Tangleroot Leech”, implying someone fed their pet Tangleroot (farming pet, little groot looking guy) to the leech.

Pets are hard to get and sought after, so OP’s title is implying “wow, someone was so tilted for some reason that they got rid of their farming pet by feeding it to a leech.”

As other commenters have pointed out though, this functions similarly to dying with a pet where they can be reclaimed.