I’m not certain if this is a bug or intended, but I’ll write about it here just to be sure.
The Horse Archer is said to be a unique unit for the Rus according to the AoE website.
In-game, they lack the marking that is on all unique units.
Example below. The Streltsy icon has the unique mark in the bottom left corner, but Horse Archer does not have it. Is this intended, or is Horse Archer uncommon such as Ribauldequin and Culverin, so we may possibly see future civs with a Horse Archer as well?