Technically, the imp isnt necessarily a "real" imp. It isn't the final form of some evil baker or low level mook. Find familiar puts a spirit of your choice (fey, celestial or fiend) into a form. Normally you only pick beasts, but the Pact of the Chain lets you pick other forms. So you could have a fey imp, a fiendish pseudodragon, or a celestial quasit (just kidding, no one ever picks quasit).