Imp

"Ah my love, I can help you. Let me be your inspiration, your heart, your vision. Let me lift you from the squalor and help you to reach the heights you so deserve. Yes, that’s it. You understand. Now, a little kiss, and the world shall be yours."

—Kwahreel, Muse

Imps are the least of the least, minor Daemons that are born from loosed emotions. In their “natural” state, they are amorphous things, endlessly cycling through a variety of shapes that seem to reflect the thought or concept that birthed them. It is only when a Dark God claims an Imp that it becomes something. Dark Magisters and Chaos Sorcerers conjure Imps to serve as Familiars.

Bubo
Imps of Nurgle are tiny versions of Nurglings, being delicate, fragile, foul, and green. They look like small withered peas but are dimpled with sphincters that issue foul yellow wetness. These creatures thrive on sickness.

Fearling
Imps of Tzeentch are small balls of pink flesh, like a miniature Horror of Tzeentch. It sits and quivers, expelling blue and pink flames from its many mouths. When it moves, it pulls itself along with pseudopodia, scorching the ground wherever it goes. Fearlings inspire rebellious thoughts and acts of anarchy.

Malice
Imps of Khorne are miniature warriors encased in black armour. They look like tiny Chaos Warriors and indeed, they have the temperament of the most vicious Khorne Champion. Malices feed on rage and hate, inspiring both in mortals.

Muse
Imps of Slaanesh are appealing, taking the form of beautiful young women with perfect nubile bodies and long hair of scintillating colours. They are tiny, no taller than three feet, but despite their size, they have power over men, stealing their souls, and draining away the capacity for sensation. But they are also inspiring, urging their victims to new heights, always pushing further and further to attain the glory they seek.

Source
Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 235)