At first, I thought you meant planetars (from D&D), and I didn’t know about that piece of trivia and felt inferior as a nerd.
But no, you really meant planarians (from reality), which is a science word and I now feel inferior because I am a moron.
Planarians are flatworms (phylum Platyhelminthes) found in freshwater bodies and their regenerative abilities have been documented for centuries (Pallas, 1766; Dalyell, 1814). Planarians can regenerate new heads, tails, sides, or entire organisms from small body fragments in a process taking days to weeks.