I thought it was rather telling that 'Q' said she was NOT what she appeared to be, and he called her an "Imp".
I think this makes for an AWESOME back story. Originating from Germanic folklore, the imp was a lesser demon. It should also be noted that demons in Germanic legends were NOT necessarily evil. Imps were often mischievous rather than evil or harmful, and in some regions, they were portrayed as attendants of the gods.
Imps are often portrayed much the same as fairies, and in some cultures, they were considered the same beings, both sharing the same sense of free spirit and enjoyment of all things fun. Note that fairies and imps are said to be possessed of all manner of supernatural powers.
It was much later in history that people began to associate fairies with being good and imps with being malicious and evil. Though imps are often thought of as being immortal, many cultures believed that they could be damaged or harmed by certain weapons and enchantments.
ALL of this could have led to some eventual revelation that beings like 'imps', and 'faeries' are NOT just ancient earth lore, but stories passed down about alien beings that visited earth from time to time.
PS - It would have been AWESOME to see some kind of showdown between 'Q' and Guinan.