A sly-panther. Any person who follows or stalks their "prey" waiting for the perfect time to strike. The slanthers presence is usually unknown to his/her victim.
Person #1: "Yo did you see Mark hiding in the bushes waiting for Justin to pass?"
Person #2: "Yaa b, I didn't even see him until he jumped on Justins back."
Person #1: "Marks such a slanther"