Shouldn't this be in the Lore thread? [/nitpick]Okay, sir grammar inquisitor.
How about this basic layout,hmm?
Nurgal: Mostly bears. Great, putrefying bears with filthy rotting claws and bloated bodys. Ranging from fun size ( cublings) to SWEETCELESTIA (renamed Ursa Major.) A focus on the aspects of a mothers love for her cubs. Expect the occasinal "Mama Nurgal". Cult members often have filthy, overgrown coats, along with rotting teeth and hooves that should not even support there weights.
Slaanesh: A more insectoid/octopid look on these guys. The main daemon would be something like a twisted version of the Kirin, with a long, thin tentical on the creatures head that could cause hallucinations to death if stung, with a twisted version of a Manticore as the Greater Daemon: With the body of a lion, the wings of a bat, and a attracive face that could pass for any gender, its legendary glamor could distract you from the long claws it has, or the hundreds of rows of sharp, serrated teeth, or even from the amorphous stinger tipped "tail" heading right for your spine.
High-ranking Cult members, besides the obvius traits, often have a strange gleam to there skin: this is due to the layer of chitin that is forming under it.
Horns might become stingers, and wings could become insectoid. Some creatures could even grow launchable spines!
Khorne. Great daemonic rhinos and metal Elephants, baby! Not much besides that they should be mostly mammals. Cult members sprouting tusks or serrated horns is not that uncommon, and having armor actualy grow on you is a very valued blessing by the god of blood. Metal wings are found in many blessed individuals already having them. These cults tend to be found in the xenos races more than ponys, for Khorne tends to favor races with the body type that can support the lifestyle ( Buffalo ahoy!), but anyone can gain the blood gods favor.
Tzeentch: I am having... issues with this one. I want the greater daemon to be a Chimara, but then I need a basic daemon. Cockatrices may be a bit too powerful, with there gaze and all, but Griffons are my prime abpony choice. Any ideas? Your the player.