Klingons,Romulans.
Klingons, Romulans, Talosians, ?The Borg? (the Planet Killer was supposedly a weapon built to destroy the Borg - non-canon source), Tribbles.
The Borg didn't appear until The Next Generation and the Planet Killer was just a rogue doomsday weapon from a war where both sides had been destroyed (nothing to do with the borg)
But these did appear
The Gorn, Tholians, Mudd, Kahn (then again in Star Trek 2),
Evil Kirk (Enemy Within) Trelane (a Q type entity).
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Star Trek: The Original Series episode airdates can be found on Memory Alpha, the Star Trek Wiki.
The star date given in the first regular series episode of Star Trek is 1312.4
Star Trek: The Original Series Star Trek: The Animated Series Star Trek: The Next Generation Star Trek: Deep Space Nine Star Trek: Voyager Star Trek: Enterprise Star Trek: Bridge Commander (TV Series)
Let that be your last Battlefield, Star Trek, Original Series.
The first Star Trek episode was called 'The Cage.' It starred Jeffrey Hunter as Capt. Christopher Pike. The pilot was rejected. The episode was created in 1966, but did not air until October 1988. Parts of this episode were aired in the original series as flashbacks in the episode 'The Menagerie', which aired in November 1966. The series was picked up with William Shatner playing the role of Capt. James T. Kirk. The first episode of what is now referred to as Star Trek TOS (the orignal series) that aired is 'The Man Trap.' It was actually the 5th episode produced. There was a pilot for Star Trek TOS called 'Where No Man Has Gone Before.' This episode was the first produced, but the 3rd to air as part of the first season of Star Trek in 1966.