From 1.17 onward this is only required Server Side.
Allows 2 players to ride one horse!
Horses have the same behavior boats, so the front player gets to steer while the back player gets a free ride!
Very easy to use, just tame a horse and have your friend right click after you hop on!
Q: I love this awesome mod, Flytre! When is it coming for forge?
A: I ain't no blacksmith, but a forge equivalent can be found here.