The Cithaeronian Lion was a lion in the mountain region of Cithaeron killed according to one version by Heracles.
Heracles since then used the lion skin as a dress and the scalp as a helmet.
Paus. 1.41.3 ff., 6, Apollod. 2.4.9, Apollod. 2.4.10
While he was with the herds and had reached his eighteenth year he slew the lion of Cithaeron, for that animal, sallying from Cithaeron, harried the kine of Amphitryon and of Thespius.
Now this Thespius was king of Thespiae, and Hercules went to him when he wished to catch the lion. The king entertained him for fifty days, and each night, as Hercules went forth to the hunt, Thespius bedded one of his daughters with him( fifty daughters having been borne to him by Megamede, daughter of Arneus); for he was anxious that all of them should have children by Hercules. Thus Hercules, though he thought that his bed-fellow was always the same, had intercourse with them all.1 And having vanquished the lion, he dressed himself in the skin and wore the scalp as a helmet.
Pausanias provides some other story about the Cithaeronia lion:
They say that Alcathous made it after killing the lion called Cithaeronian. By this lion they say many were slain, including Euippus, the son of Megareus their king, whose elder son Timalcus had before this been killed by Theseus while on a campaign with the Dioscuri against Aphidna. Megareus they say promised that he who killed the Cithaeronian lion should marry his daughter and succeed him in the kingdom. Alcathous therefore, son of Pelops, attacked the beast and overcame it, and when he came to the throne he built this sanctuary, surnaming Artemis Agrotera (Huntress) and Apollo Agraeus (Hunter).
Such is the account of the Megarians; but although I wish my account to agree with theirs, yet I cannot accept everything they say. I am ready to believe that a lion was killed by Alcathous on Cithaeron, but what historian has recorded that Timalcus the son of Megareus came with the Dioscuri to Aphidna? And supposing he had gone there, how could one hold that he had been killed by Theseus, when Alcman wrote a poem on the Dioscuri, in which he says that they captured Athens and carried into captivity the mother of Theseus, but Theseus himself was absent?
Pindar in his poems agrees with this account, saying that Theseus, wishing to be related to the Dioscuri, carried off Helen and kept her until he departed to carry out with Peirithous the marriage that they tell of. Whoever has studied genealogy finds the Megarians guilty of great silliness, since Theseus was a descendant of Pelops. The fact is that the Megarians know the true story but conceal it, not wishing it to be thought that their city was captured in the reign of Nisus, but that both Megareus, the son-in-law of Nisus, and Alcathous, the sonin-law of Megareus, succeeded their respective fathers-in-law as king.
It is evident that Alcathous arrived from Elis just at the time when Nisus had died and the Megarians had lost everything. Witness to the truth of my statements the fact that he built the wall afresh from the beginning, the old one round the city having been destroyed by the Cretans. Let so much suffice for Alcathous and for the lion, whether it was on Cithaeron or elsewhere that the killing took place that caused him to make a temple to Artemis Agrotera and Apollo Agraeus.