There are four major Chaos Gods in Warhammer. Each of them has it's own main idea. And each of them differs so much from others, that it's hard enough to put all of them to one army. But the most separated from others is Nurgle.
Khorne means rage, blood. It is not hard to give horses or other mounts to bloody berserkers. Tzeentch is hope - he is bad only because he is opposite to imperium or Emperor, so he can do everything. Slaanesh is pleasure - nothing evil, just pleasure. But Nurgle is decay. How many animals will accept decay? How many possibilities decay have? Medieval horses were scared by smell of camels, what can we say about Nurgle warriors? Griffins, dragons - who will take decayed champion close to himself? The only way to get horses or other mounts for Nurgle warriors - put decay deep in their minds and bodies. Small job for major Chaos God, but almost impossible for his cults. Only humanity is decayed enough to accept Nurgle without compromises. This guys far from "grean peace", in spite of being green to.