A342
PHB page 198 (under Mounting and Dismounting):Arguably because of the wording "knocked prone" you wouldn't have to make the saving throw if you intentionally drop yourself prone, but it is definitely possible to be prone while mounted.If an effect moves your mount against its will while you're on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. If you're knocked prone while mounted, you must make the same saving throw.