The cavalry......
Good question, I'm hoping they're another community that is filled with good people who know how to fight.....
The guys with the horses are from a community called The Kingdom. They ride on horseback and wear armor (of a sort) like from medieval times.
***SPOILERS...sort of***
They are part of The Kingdom. According to the comic book, they are in a school in the Washington DC area.
In the comic, Rick and somebody named Paul convince Hilltop, The Kingdom and Alexandria to join forces to fight Negan and the Saviors.
Before they can attack the Saviors, Negan attacks Alexandria, burning/bombing it to kingdom come (heh, heh, see what I did there?)
Anyway, the homeless Alexandria-ites and the Kingdom gather in Hilltop to make a giant army and fight and defeat Negan and the Saviors.
TAKE THIS WITH A GIANT GRAIN OF SALT...the comic and show are different in many, many ways, but I think we can take from this is that The Kingdom are good guys.