So no matter what type of familiar, bat, cat, imp, or pseudodragon a familiar is a spirit that is celestial, fey, or fiendish in origin. You don't ever get a regular imp or quasit even if your warlock chooses to have the origin be fiend, other true imps probably make fun of your imp.
One of the newer 5E adventures has a regular pseudodragon that can potentially bind with a PC. Checking for it now.
Edit: Severin's pseudodragon in the Rise of Tiamat is NE and will try to kill the PCs if you kill Severin, but I think the potential is there for perhaps an evil warlock PC or wizard to win it over. At least, I'd potentially have that as an option for exceptionally clever, good roleplayers.
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