Chatterfang, Squirrel General
Legendary Creature — Squirrel Warrior
3 / 3
Forestwalk (This creature can't be blocked as long as defending player controls a Forest.)
If one or more tokens would be created under your control, those tokens plus that many 1/1 green Squirrel creature tokens are created instead.

, Sacrifice X Squirrels: Target creature gets +X/-X until end of turn.
You don't need to control the spell or ability that creates the tokens, nor do you have to be the one creating the tokens for Chatterfang's ability to apply. As long as the tokens are being created under your control, Chatterang's replacement effect will apply.