Introducing Poke Ride in Pokemon USUM (Ultra Sun and Ultra Moon)! This article explains what kind of feature Poke Ride is.
What is Poke Ride?
Means of Transportation Using Pokemon

Poke Ride is a new feature that allows you to ride Pokemon to reach places inaccessible by human strength alone. Using Poke Ride, you can travel across land, sea, and sky to various locations.
They Don't Become Party Pokemon
Monsters usable for Poke Ride cannot be used as party Pokemon. You obtain them through in-game events and can only summon them when needed for travel.
HM (Hidden Machines) Do Not Exist
In generations before Sun and Moon, Poke Ride didn't exist; instead, HMs were used to travel by sky or sea. Since HMs were abolished in Sun and Moon, it's believed that HMs do not exist in Ultra Sun and Ultra Moon either.
New Poke Rides Appear!
A Minigame with Mantine Surf?

In Pokemon USUM (Ultra Sun and Ultra Moon), "Mantine Surf" will be added, allowing you to enjoy surfing on Mantine's back. Mantine Surf seems to be not only a means of transportation between islands but also a competition to achieve high scores.
Shortcut Keys
Normally, you need to press the Y button to open the Ride Pokemon list and then select the Pokemon to use. However, by following the steps below, you can activate a Ride Pokemon simply by pressing the D-pad.
1.Press the Y button to open the Ride Pokemon screen.
2.Tap the "○" on the left side.
3.Register the specified Ride Pokemon to each direction on the D-pad.
4.Afterward, you can use the corresponding Ride Pokemon by pressing the registered D-pad direction.
How to Use Poke Ride
Tauros (Charge)
You can travel faster than on foot. Not only that, but it can also break obstacles like rocks, so there are places you can't proceed without using Tauros.
Stoutland (Search)
It allows you to perform "Dowsing" as in past games. By pressing the B button while moving, you can find hidden items invisible to the eye.
Mudsdale (Dash)

With Poke Ride, you can traverse gravel paths that are impassable by human strength.
Lapras (Swim)
You can travel on water using Surf.
Sharpedo (Jet)

Unlike Lapras's Surf, it can destroy rocks in the sea. If there are rocks and Lapras cannot proceed, use Sharpedo.
Charizard (Fly)

You can fly on Charizard's back. In Sun and Moon, you could only travel to predetermined points.
Machamp (Push)
It can move large rectangular rocks. There are also places where you cannot proceed unless you move them.