Can a centaur (from Unearthed Arcana: Centaurs and Minotaurs) with a rider use Stealth/move stealthily?
There is nothing that prevents it; however, if the centaur uses Stealth and the rider doesn’t, the centaur will be detected immediately (barring specific circumstances where the rider is only visible from the waist up) and vice-versa.
The two must perform a Group Check (p. 175 of the Player's Handbook):
When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check. In such a situation, the characters who are skilled at a particular task help cover those who aren't.
To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds. Otherwise, the group fails.
The centaur and the rider must both make a Dexterity (Stealth) check, and the highest result is the one that is used. This actually makes it easier for the two of them to hide than if either were alone - presumably this is "the characters who are skilled at a particular task help cover those who aren't" in action.
I would say that two creatures sneaking together in this way are not really a "group check" because you are not doing the same thing -- one is walking while the other is riding on him. Therefore you have to both make Stealth checks individually, so, that means not just the centaur but also his/her rider. The PHB says "Make a Dexterity (Stealth) check when you attempt to conceal yourself from enemies" and well, if you are riding the centaur, both of you are attempting to conceal yourself but in a different way and in different respects.
It makes sense that the rider still has to behave stealthily e.g. not twitching or clearing his throat, etc. But the DM might determine that the DC is lower for the rider since he/she is spared some elements of stealthy movement, such as making sure not to snap a twig with a footstep.