In 2141, the Horizon encountered a Deltan ship. Travis Mayweather, who was fifteen at the time, described the group of Deltans as "very attractive" and "very open". (ENT: "Bound") The Horizon once came under attack by Nausicaan pirates; it barely escaped in one piece. (ENT: "Fortunate Son")
In January 2153, the Horizon was attacked by the same pirates that attacked the ECS Constellation many weeks prior. Mayweather managed to upgrade the ship's plasma cannons by connecting them directly to the ship's impulse engines. This, combined with his expert piloting skills, allowed the Horizon to withstand the attack. (ENT: "Horizon")
Among the planets visited by the ECS Horizon were:
The Horizon made only a single appearance, in "Horizon". In the final draft script of that episode, the craft was described as "space-worn," as of 2153. The ship was rendered as a CGI model, designed by John Eaves and built by Brandon MacDougall at Eden FX.
The assignment patch for the Horizon was created by Anthony Fredrickson, a design task provided to him by Michael Okuda. "Because this was the Horizon, I did a star dropping behind the horizon and an old-time sailing ship with a star glowing at the top of its mast," Fredrickson recalled. (Star Trek: Communicator issue 144, p. 36)
According to the final draft script of "Fight or Flight" (dated 1 May 2001), the Horizon made a two light year journey from Andoria to a planet called Bylaran Prime. In a revised final draft of the episode's script (dated 27 June 2001), Horizon's starting point in this trip was renamed to a newly invented planet called Lavinius. The scene in which the ship was referred to as having undertaken its voyage between Lavinius and Bylaran Prime was filmed but then deleted. The deleted scene can be found on ENT Season 1 DVD.
According to the Pocket ENT novel Kobayashi Maru, the ECS Horizon was the starship Horizon that visited Sigma Iotia II, as mentioned in "A Piece of the Action". The fate of the Horizon was unclear; by the end of the book, it was deemed missing and presumed destroyed by Romulan action. This connection to the Original Series episode was also suggested by the set decoration of the ship in "Horizon", as a hard-bound copy of a book beginning with the title Chicago Gangs can be briefly glimpsed on a bookshelf in Travis' quarters aboard the ship.