Sources : Hyena
Aesop's Fables [6th century BCE +] (The Hyena and the Fox; Perry 242) They say that hyenas change their nature every year, so that sometimes they are male and sometimes female. So when a hyena saw a fox and criticized her for having spurned her friendly overtures, the fox replied, 'Don't blame me! Blame your own nature, which makes it impossible for me to tell whether you would be my girlfriend or my boyfriend!' [The Two Hyenas; Perry 243] They say that the hyena has a double nature: for a period of time the hyena is male, and then later on she is female. The story goes that when a male hyena was treating a female badly, she said to him, 'Listen here: remember how things used to be, and don't forget that I will be a male hyena the next time around!' - [ Gibbs translation]
Aristotle [ca. 350 BCE] (De animalibus Book 6, chapter 28.2): It is a mistake to say that each individual has the sexual organs of both sexes. That of the male resembles the same organ in the wolf and the dog. That which has been imagined to be the female organ is placed beneath the tail, and it resembles that of the female, but is imperforate, and the anus is beneath it. The female hyena has an organ similar to that which bears its name in the male. It is placed beneath the tail, and is imperforate. Beneath this is the anus, and below this again the true genital organ. [Book 8, chapter 7.2] It also secretly attacks men, and hunts them down; it hunts dogs also by vomiting like men; it also breaks open graves for the sake of this kind of food.- [Cresswell translation, 1887]
Ovid [1st century CE] (The Metamorphoses, Book 15: 391-417): If there is anything to marvel at, however, in these novelties, we might marvel at how the hyena changes function, and a moment ago a female, taken from behind by a male, is now a male. - [Kline translation]
Pliny the Elder [1st century CE] (Natural History, Book 8, 30): Hyenas are like a cross between a dog and a wolf; they break everything with their teeth, swallow it at a gulp, and masticate it in the belly. (Book 8, 44): Hyenas are believed to become male and female in alternate years. It cannot bend its neck, which extends all along its backbone, so it must turn its whole body. It can imitate the human voice; it does this outside a shepherd's outside, calling him by name so he comes outside, where the hyena can tear him to pieces. It can also imitate the sound of a person becoming sick, which it does to attract dogs so it can attack them. Only the hyena digs up graves to eat the corpses. Its eyes have many colors, and dogs are struck dumb if its shadow falls on them. Any animal that a hyena looks at three times will be unable to move.
Aelianus [170-230 CE] (On the Characteristics of Animals, Book 1, chapter 25): Should you this year set eyes on a male hyena, next year you will see the same creature as a female; conversely, if you see a female now, next time you will see a male. They share the attributes of both sexes and are both husband and wife, changing their sex year by year. [Book 6, chapter 14] The Hyena, according to Aristotle, has in its left paw the power of sending to sleep and can with a mere touch induce torpor. For instance, it often visits stables, and when it finds any creature asleep it creeps softly up and puts what you might call its sleep-inducing paw upon the creature's nose, and it is suffocated and overpowered. Meantime the hyena scoops out the earth beneath the head to such a depth as makes the head bend back into the hole, leaving the throat uppermost and exposed. Thereupon it fastens on to the animal, throttles it, and carries it off to its lair. And it attacks dogs in the following manner. When the moon's disc is full, the hyena gets the rays behind it and casts its own shadow upon the dogs and at once reduces them to silence, and having bewitched them, as sorceresses do, it then carries them off tongue-tied and thereafter puts them to such use as it pleases. [Book 7, chapter 22] At any rate the hyena prowls about cattle-folds by night and imitates men vomiting. And at the sound dogs come up, thinking it is a man. Whereupon it seizes and devours them. - [Scholfield translation]
Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 27.23): Africa also gives forth hyenas. The necks of these creatures are stiffened with a spine which is one continual unit, and they cannot turn except by an entire revolution of the body. Hyenas possess many marvellous features. First, they range around shepherds’ huts and learn, by assiduous listening, to mimic the human voice, so they may savage men called out in the night by their cunning.  They also imitate the sound of people vomiting, and thus devour the dogs attracted by their false retching. If by change dogs out hunting touch the shadow of a hyena while pursuing him, they lose their voices and cannot bark. Hyenas, by way of spying out buried bodies, dig up tombs. It is an easier matter to capture a male hyena, for in the females a more crafty cunning is instilled by birth. Their eyes change colour and have a complex variety.  In the pupils of hyenas a stone is found which they call “hyaenia”. It possesses this power: when placed under the tongue of any man whatever, he prophesies the future. Any animal which the hyena walks around three times cannot move. For this reason, they proclaim that the hyena has magical knowledge. - [Arwen Apps translation, 2011]
Isidore of Seville [7th century CE] (Etymologies, Book 16, 15:25): Hyaenia is a stone found in the eye of the hyena and they say that if it is placed under the tongue of a man he foretells the future.
Aberdeen Bestiary [circa 1200 CE] (folio 11v-12r):There is an animal called the hyena, which inhabits the tombs of the dead and feeds on their bodies. Its nature is that it is sometimes male, sometimes female, and it is therefore an unclean animal. Since its spine is rigid, all in one piece, it cannot turn round except by turning its body right around. Solinus recounts many marvellous things about the hyena. First, it stalks the sheepfolds of shepherds and circles their houses by night, and by listening carefully learns their speech, so that it can imitate the human voice, in order to fall on any man whom it has lured out at night. The hyena also [imitates] human vomit and devours the dogs it has enticed with faked sounds of retching. If dogs hunting the hyena accidentally touch its shadow behind, they lose their voices and cannot bark. In its search for buried bodies, the hyena digs up graves. The sons of Israel resemble the hyena. At the beginning they served the living God. Later, addicted to wealth and luxury, they worshipped idols. ... This beast has a stone in its eyes, called hyenia; anyone who keeps it under his tongue is believed to foretell the future. It is true that if the hyena walks three times around any animal, the animal cannot move. For this reason men declare that the hyena has magical properties. In a part of Ethiopia the hyena mates with the lioness; their union produces a monster, named crocote [leucrota].