A horse can be pretty expensive to get, and tedious to transport in areas with narrow entrances. I am wondering if a friendly Druid wild shaped into a horse can be used as a mount, especially since the Mounted Combatant feat gives so many advantages to a mounted warrior.
Yes, you can mount your Druid friend
The rules for Mounts indicate that a willing creature at least one size category larger than you, with appropriate anatomy, can serve as a mount. A horse is obviously an acceptable mount, so a Druid Wildshaped into a horse is acceptable, so long as they are willing.
Do note that, because the Druid is intelligent, it must act as an Independent Mount, rather than a Controlled Mount. This means it will retain its position in initiative order and act normally, despite the rider.
Not only do the rules clearly allow for it, it is extremely powerful.
A paladin riding a moon Druid with the right feats and abilities can wreck a monster way above it’s class. Why? Because with the Mounted Combat feat you can have advantage on every attack as long as the Druid can make itself bigger than the target.