If you sacrifice a nontoken creature you control but don't own, Prowling Geistcatcher will still cause you to exile that card from its owner's graveyard. The card will be returned to the battlefield under your control with the rest of the exiled cards when Prowling Geistcatcher's last ability resolves.

If you sacrifice a creature token you control but don't own, you will still put a +1/+1 counter on Prowling Geistcatcher.