They're just demonic goblins.
I mean, I have read some novels where the demonic beings were different, but in general 'Imps' are just the weakest form of demon, equivalent to goblins for monsters.
Weak, small, red instead of green, have a tail and, sometimes, bat wings.
I guess, if demons evolve in the novel, you could make them unique by making them able to evolve into any other form of demon instead of being locked into a single evolution line.
Or, depending on the tipe of novel, you could make them unique by NOT making them goblins... make them the closest equivalent to a human amongs evil demons, capable of using all types of magic and being good.