The Axanar had green blood. Their bodies produced triglobulin from the zymuth gland. This substance had medical and aphrodisiac properties, and was used by a number of different species for those purposes. Some even forcefully "harvest" Axanar for their triglobulin.
In 2151, the Axanar made first contact with Humans via Enterprise NX-01, after an unnamed species attacked an Axanar cargo ship. When Enterprise sent an away team over to the cargo ship to investigate, the team found that the crew were now mutilated corpses. The Axanar returned, responding to a distress signal set by the Enterprise crew, but a translator malfunction led them to believe that Enterprise itself was responsible. Shortly afterwards, however, they were able to learn from Hoshi Sato, having translated the Axanar language, that the true attackers were the ones attacking Enterprise. The Axanar responded by offering assistance. (ENT: "Fight or Flight")
Sometime prior to 2152, an Axanar starship stopped by the automated repair station. One of the Axanar crewmen was secretly added to several humanoid hosts aboard the station, whose brains had been tied to the station's computer. (ENT: "Dead Stop")
By the mid-23rd century, a conflict had broken out at the site of the Axanar's homeworld, involving the United Federation of Planets, Starfleet, and Captain Garth of Izar. Garth was victorious in what became known as the Battle of Axanar, and his exploits in the action became required reading at Starfleet Academy. James T. Kirk participated in a subsequent peace mission at Axanar, for which he was awarded the Palm Leaf of Axanar Peace Mission. (TOS: "Court Martial", "Whom Gods Destroy")
Reference in a holodeck simulation
The Axanar were mentioned in a holodeck simulation that, in 2367, Commander William T. Riker was fooled into believing was reality. In the simulation, the species was referred to in Riker's future service record, which included a note about the Axanar. They had hosted a conference on Beta Quadrant exploration and Federation security that had been held at the Deneva Special Congress on Interstellar Affairs. On stardate 52384, Riker, then captain of the USS Enterprise-D, had been selected for recognition as the special Federation representative to this conference. (TNG: "Future Imperfect", okudagram)
In the script of "Fight or Flight", the name of this species was phonetically notated to be pronounced "AX-a-nar".
For the remastered episode of "Future Imperfect", the original text of Riker's service record was replaced.
In the RPG book The Four Years War, the Axanarians were described as pre-industrial humanoids with some genetic similarities to Klingons.
In the RPG book Star Trek: The Original Series - Core Game Book, the Axanari were tall humanoids with a white secretion covering their skin and wide range of hair colors including blue, green, red and orange. The Axanari have superior perception and pattern recognition abilities that sometimes cause them to see patterns that aren't there. When asked a question, an Axanari will always have an definite answer, even when there is insufficient evidence, and will be absolutely certain that he is correct. The Axanari applied for Federation membership in 2252, but the Centaurans and Vulcans wanted to reject their application due to their rigid caste system and inferior technology. The Axanari decided to impress the Federation by conquering several nearby planets and presenting them to the Federation. This led to a short war with the Federation. At the conclusion of the war, the Axanari abandoned their conquered planets and their leaders resigned in disgrace. They eventually became members of the Federation.