who were the people dressed like cavalry behind trump

They were dressed as the Rough Riders — the famous cavalry unit associated with Theodore Roosevelt. The escort was part of Trump’s visit to Medora, North Dakota, for the opening of the Theodore Roosevelt Presidential Library.

What that meant

The Rough Riders were Roosevelt’s volunteer cavalry unit from the Spanish- American War, so the horseback riders were a historical reenactment rather than a military escort.

Why people noticed

The visual was meant to evoke Roosevelt’s image and legacy, which is why it stood out so much online.

TL;DR

Those people behind Trump were Rough Riders reenactors on horseback , not actual cavalry.