Ride Along

Ride Along allows villagers and smaller animals to ride larger animals. "Small" and "large" are calculated automatically based on hitbox size, and should work reasonably well for modded entities.

This allows you to do various things, including:

 - Put a villager on a horse and lead him somewhere

 - Get animals into and out of boats more easily

 - Bring your cat somewhere on horseback, if you have a mod such as Realistic Horse Genetics that lets horses carry two riders

    - This also works for bringing home on horseback that one rare Genetic Animals chicken you found

There are two methods to mount your entities!

1. While riding a mount or vehicle that can carry more than one passenger (such as a boat), Ctrl + right click on the rider
2. Attach the mount to a lead, then Ctrl + right click on the rider

To dismount all passengers, simply Ctrl + right click on the mount.

By default the key to use is Left Control, but it's rebindable to whatever your heart desires.

Bringing a cat on an adventure, with a horse from Realistic Horse Genetics. Most RHG horses will let you bring a small companion, as long as the horse is not too small itself.

If you are a mod author and want to adjust how much an entity can carry or how big a creature has to be to carry it, see Notes for mod authors

History
In Minecraft 1.15 and 1.16, this was a feature of Realistic Horse Genetics (and if you want it on those versions, you should use that mod instead). Starting with Minecraft 1.17, Ride Along became its own, independent, mod.