In my creative universe, there exist several different kinds of sprites and faeries. Imps are one such faerie and this particular imp is named Widdershins. The imps are the charges of a nearly forgotten old pagan forest god.
The little one on his tail, and in this artwork, [link]
is what Widdershins normally appears as. Under special circumstances; namely, if someone makes a very large and sincere sacrifice to his deity, and there is a greater task that needs to be done than what Widdershins's normal tiny body can do, his deity may grant Widdershins this full-grown form using the energy that was present in the sacrifice.
Widdershins isn't evil, though imps like to play tricks on people, and devils are associated with avarice and pandemonium. Basically, they test the inner strength of humanity.