Serpents are snakes. Often they are either very large snake-like creatures such as sea serpents or poisonous snakes like vipers, rattlesnakes or cobra. In the Ancient Ones' alphabet the serpent is represented by the letter U.
Mordack's island had statues of stone serpents that could kill with their glance.
Throughout history serpents have been depicted as vile, evil creatures. This also seems to be true in the world of Daventry, evidenced by the satanic serpentine icons and statues in the dark wizard Mordack's castle, and by the fact that Alexander refers at many times to Alhazred as "that snake".