Fear/Terror. If a character is riding a creature causing Fear/Terror, then the entire combined model of rider and mount is assumed to cause Fear/Terror.
Stupidity. If either the rider or the steed suffer from Stupidity, test using the rider's Leadership - both are affected if the test is failed. If both the rider and the steed suffer from Stupidity, test only once for both.
Immunity to Psychology or Panic, Hatred, Frenzy, Stubborn, Unbreakable. If either the rider or the steed is subject to any of these rules, the whole combined mode is. This means that all the parts of the model (a monstrous mount, or the crew and steeds pulling a chariot) are affected by these rules.