When attacking a mounted target in melee, the attacker may choose whether to target the mount or the rider. In the case of unintelligent creatures where there is no clear choice, a random target can be determined with a d6:

When attacking a mounted target with missile fire, a random target should always be determined in the above way.

This d6 roll works in cases of a M sized rider and a L sized mount, such as a PC and a horse. The die roll should be adjusted appropriately in cases of a larger size category difference, taking into note the rules in the DMG for the random determination of missile targets on DMG 63.

In cases where height is a significant factor, that should be taken into account. For example, a kobold with a short sword attacking a man riding a large, old, red dragon should always target the dragon.