The backwards-compatible version of the original Forza Horizon game has now been enhanced for use on the Xbox One X console.
As with existing 'Xbox One X Enhanced' Xbox 360 games, the boost to Forza Horizon is primarily focused on upgrading the visual quality. On top of improving the aliasing effects, the Xbox One X-supported version of the original Forza Horizon also boasts increased texture detail and a 4K resoultion.
Alternatively, Forza Horizon players can also select a 'Performance' mode in the console's 'Settings' menu. Specific performance improvements weren't cited in the official announcement, though it was confirmed the visual fidelity in this mode is comparable to the Xbox One and Xbox One S versions of Forza Horizon.
The decision to enhance the original Forza Horizon for the Xbox One X is a peculiar one, considering the game was delisted from the Xbox Games Store in October 2016. At time of writing, it's unclear if this One X enhancement is indicative of an upcoming re-release for the original Forza Horizon.