Steel Horse is a motorcycle manufacturer in the HD Universe.
Its name doesn't appear on motorcycles, but the police on the police radio will refer to a motorcycle as one. The only known vehicle made by Steel Horse (so far) is the Zombie. It could be based on American IronHorse, judging from the name. Steel Horse is an old slang term for motorcycles used by biker gangs.
While there are no playable Steel Horse vehicles in GTA V, the office of Beeker's Garage in Paleto Bay features a manual for a Steel Horse featuring Steel Horse logos, and a similar poster. As the Hexer in-game is a custom bike made by the chop shop Liberty City Cycles, it is possible that the standard Hexer in manufactured by Steel Horse.