Mr. Apothecary is an old store merchant that makes potions for all witches and warlocks.  He chases the witches that "pop in" - mostly Samantha.  He was first seen in "Samantha's Lost Weekend."  

His real name is Postlethwaite, not Mr. Apothecary, as can be seen just before cutting to the inside of the apothecary we see outside a sign posted "Postlethwaite's Potent Potions" in the episode "A Plague on Maurice and Samantha."