What it does
I'm not aware of "Rat Tail" being an official Magic: The Gathering keyword or ability. It doesn't correspond to any printed keyword mechanic that I can verify.
It's possible you're thinking of something else—perhaps a specific card name, a creature type interaction (like Rats with cards such as Relentless Rats or Marrow-Gnawer), or a custom/unofficial mechanic from a fan set or homebrew. Mythicwyrm may have community-created content that uses this term, but I can't confirm its rules text without more information.
If you can share the card or the reminder text where you saw "Rat Tail," I'd be glad to break down what it does, the common rules pitfalls, and how it plays out in Commander. I'd rather not invent mechanics and risk giving you incorrect information.
Iconic cards with Rat Tail
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