Definition
A piece of downloadable content or microtransaction for a video game, especially one purely cosmetic and considered being of bad value.
Origins
In reference to Bethesda Game Studios' 2006 RPG, The Elder Scrolls IV: Oblivion, which notoriously offered horse armor as a paid piece of DLC.
How People Actually Use It
The first downloadable content available for the Xbox 360 game Oblivion. Upon its release, there was an uproar in the gaming community due to its nearly useless feature set and and relatively high price tag ($2.50 US). Since then, the term horse armor has been used to describe video game features that are useless or overpriced.
"Can you believe they're charging $10 extra for the horse armor in Madden 07: Hall of Fame Edition? It seems like every year video game developers expect us pay $50 for a sequel and all they've added is some horse armor."
Source: Urban Dictionary