During the War
Mr. Horse founded Robronco, which would become Equestria's main manufacturer in robotic technologies. Horse and his company developed a wide range of robots for commercial and military service. Mr. Horse worked closely with the Ministry of Technology & Stable-Tec. Horse was employed/commissioned by Goldenblood to take on some personal project for him, including a study of star metal found beneath Hoofington. This led to work on the Tokomare, an ancient star metal reactor buried underneath Hoofington that, theoretically, had the potential to produce enough electrical energy to power not just Equestria but the entire world several times over, making the energy supply problems that started the war moot. Some on the research team hoped the abundance of cheap power created by the Tokomare would make the zebras surrender and buy their energy from Equestria. Horse seemed to have ambitions to use the Tokomare for powering more than just "light bulbs."
Horse blackmailed Goldenblood with footage of a murder Goldenblood committed beneath Hoofington so that he could get a stake in the OIA's secret projects. Moments after Goldenblood agreed, both learned of the failed assassination of Applejack, and Goldenblood made Horse interim director of the Ministry of Technology while Applejack recovered. During that time, Horse uncovered evidence of the huge scale of Goldenblood's secret projects that Applejack hadn't noticed. It is not clear whether Horse was directly or indirectly involved with the assassination attempt.
When Goldenblood was arrested for treason and removed as the director of the Office of Interministry Affairs, Luna appointed Horse as interim director. However, it seems that senior OIA management was still loyal to Goldenblood, and may have secretly been running the OIA to Goldenblood's orders during Horse's brief tenure.
At some point prior to taking over Goldenblood's position, Horse built a ripoff of the Crusader Maneframe, able to copy a person's mind/personality instantly. He used his demonstration to try and copy Goldenblood's mind, but the latter refused to co-operate willingly. Goldenblood's associate Eclipse was caught in the machine's copying powers, creating a copy of her mind. Eclipse was in fact an undercover Princess Luna, and her copied mind took on a more Nightmare Moon-esque personality and eventually became Cognitum. Horse thought he could use Cognitum's secrets to his advantage and worked with her to remove Goldenblood from power.
On the Last Day, Horse was exposed to Enervation and survived in a stasis pod. However, the damage was done and according to Cognitum, his mind was destroyed, leaving him a hollow shadow of his former self, surviving in motionless agony for two-hundred years. Cognitum shows him off to Blackjack as one of her "trophies", using his Sweetie Bot companion as an on-and-off host. When she addresses Horse, he responds with only a physical shudder.
Horse was described as young looking and energetic.
Horse had an almost childlike enthusiasm concerning new and rare technology/sciences. Horse was also manipulative and a very successful entrepreneur, not above blackmail. He was also very forward thinking, laying contingency plans should the Ministry of Morale ever raid his place of business by having emergency exits and secret rooms built. Horse once proposed to Sweetie Bell, but she turned him down. Horse wanted a Sweetie Bell that wouldn't say no, and thus built an equine robot named Sweetie Bot that replicated the appearance, voice, mannerisms, and personality of Sweetie Bell.
Besides his vast wealth and influence, Mr. Horse also owned one of the extremely rare Star Blasters.
Mr. Edwin Robert Horse is the undisputed master of New Pegas, a pre-War city, which he controls through a network of "families" backed up by a small army of robots. Although he has never been seen by anypony in centuries except via a computer monitor, his appearance and voice are identical to the legendary Robronco founder, and he identifies himself as such. It is not known if this is the original Horse, a clone hiding behind a screen, or simply a computer simulation of some sort.
When his courier is murdered and his property stolen by a trusted lieutenant, aided by a gang known as the Cossacks, Horse hires the services of an unsociable bounty hunter called "Dead-Shot" to retrieve it. Later, Horse hires Dead-Shot on a more permanent basis, to perform as a proxy negotiator with minor factions in and around the New Pegas area.