If a cavalier is riding his mount when combat starts, which initiative is used, his or the mount's? Or do they both roll?
You use the Cavalier's initiative.
Ok, thank you.
Mounted Combat
These rules cover being mounted on a horse in combat but can also be applied to more unusual steeds, such as a griffon or dragon.Mounts in Combat: Horses, ponies, and riding dogs can serve readily as combat steeds. Mounts that do not possess combat training (see the Handle Animal skill) are frightened by combat. If you don't dismount, you must make a DC 20 Ride check each round as a move action to control such a mount. If you succeed, you can perform a standard action after the move action. If you fail, the move action becomes a full-round action, and you can't do anything else until your next turn.
Your mount acts on your initiative count as you direct it. You move at its speed, but the mount uses its action to move.
Ok. So... if the cavalier makes a charge attack, that uses the mount's action, correct? If the cavalier doesn't make a movement, then both can make an attack, as long as the target it within the mount's reach.
Cavalier mounts are always considered Combat Trained, so there's no need for a Ride check.