The Hyena’s Alternating Gender

Aelian, History of Animals 1.25

“if you should at any time see the male Hyena, the next time you see the same animal it will be female. If you see the female first, then later you will see the male. They have the aspects of sex in common: they marry and are married and they alternate their nature every year. Therefore, this animal has demonstrated the antique nature of Kaineus and Teiresias not with wild tales but with the fact themselves.”

Τὴν ὕαιναν τῆτες μὲν ἄρρενα εἰ θεάσαιο, τὴν αὐτὴν ἐς νέωτα ὄψει θῆλυν· εἰ δὲ θῆλυν νῦν, μετὰ ταῦτα ἄρρενα· κοινωνοῦσί τε ἀφροδίτης ἑκατέρας, καὶ γαμοῦσί τε καὶ γαμοῦνται, ἀνὰ ἔτος πᾶν ἀμείβουσαι τὸ γένος. οὐκοῦν τὸν Καινέα καὶ τὸν Τειρεσίαν ἀρχαίους ἀπέδειξε τὸ ζῷον τοῦτο οὐ κόμποις ἀλλὰ τοῖς ἔργοις αὐτοῖς.

An explanation:

That's likely due to the female spotted hyena's pseudopenis – a clitoris that also contains the ureter & vagina, as well as flaps of skin resembling a scrotum. Mating & childbirth are difficult to say the least: over half of firstborns die in birth, as do 15% of first mothers.

— Katusagios Göranssen (@SeadogDriftwood) December 29, 2018