Comments from the Finger: Originally conceived as a Mammoth Rider by Rex Gunn, this class took a distinctly mounted-combat approach when the pieces came together, and I quite like the result.
Circle of Beast Riders
The Circle of Beast Riders is composed of druids that walk in lock-step with the beasts of the wild, and learn to ride them, no matter how ferocious the beast. These druids travel far afield on the backs of wild creatures to defend the beasts of the wild from those that might harm them. And in combat, they are a terrifying force; where a knight may approach on the back of a warhorse, a beast rider will arrive atop a wild bear, wolf, or mammoth.
Starting when you choose this circle at 2nd level, you have a fearsome mount that carries you into combat. Your mount must be a Large beast, cannot fly, and can have a maximum CR of 1. You can control this mount even if it hasn't been trained to accept a rider. Its hit point maximum equals its normal maximum or four times your druid level, whichever is higher.
The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it. All other rules of mounted combat apply.
If your mount dies, you can obtain another one by spending 8 hours magically bonding with another beast that isn't hostile to you, either the same type of mount as before or a different one.
Starting at 6th level, you can tame a mount with a challenge rating as high as your druid level divided by 3, rounded down. At 15th level, you can also tame Huge mounts.
At 2nd level, you can mount and dismount a creature without spending any movement.
You can also use your action to command your mount to take the Charge action. When a mount makes a Charge, it moves at least 10 feet in a straight line and can make one melee weapon attack against a creature within its reach. Additionally, when you command your mount to Charge, you can make one attack as a bonus action.
At 6th level, your mount's attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage as long as you are riding it.
Additionally, whenever you cast a spell which affects only yourself, you can choose for it to also affect your mount.
Mount Wild Shape
Starting at 10th level, your mount learns how to assume the shape of other beasts. As an action, you can expend two uses of Wild Shape to transform your mount into another Huge or smaller beast with a CR less than or equal to its own. Unlike your normal mounts, your wildshaped mount can fly, if its new form allows it to. Your mount follows all the rules of Wild Shape while transformed.
At 14th level, you can cast the spell enlarge/reduce spell on your mount at will.
Changelog: 10/14/16: Circle Mount: Begins at a CR of 1, its HP is equal to 4 times your druid level, if it was lower
Caviler: Bonus action to attack after a charge moved to this feature
Mounted Charge: Bonus action to attack removed. Advantage on concentration added. Renamed Mounted Casting.
Beastial Fury: Cut
Mount Wild Shape: Moved back to 10th level
Mammoth Rider: Added at 14th level
10/15/16: Mounted Casting: Concentration advantage removed. Extended casting added.