Imps were not a separate type of creature in Tolkien's world, but rather a smaller and relatively harmless being of any kind. The smallest, least powerful Orcs were referred to as Goblin-imps, at least by Gollum, and the Mouth of Sauron used the same word to refer to Hobbits. The word imp comes originally from an Old English term for a young plant shoot, and from that source developed to imply a young person or child (and only later a mischievous supernatural being).