During my formative years I recall being instructed and/or quizzed on the names given to different genders in animals. When hearing a question on a television quiz show recently, I wondered when and why swans were classified as cobs and pens. Before long I was looking into the origins of boar and sow, bull and cow, buck and doe and all the rest.
Bull and Cow
Not only cattle but camel, elephant, elk, giraffe, hippopotamus, walrus, whale, and even the termite, rat, crocodile are known as bulls and cows. The word 'bull' can be traced to a Proto-Indo-European word bhel meaning 'to inflate, swell', thereafter a Germanic root meaning 'to roar', and ultimately used for male cattle. Not until the early 17th century did the males of other creatures come to be known as 'bulls'. The female 'cow' comes from Proto-Indo-European gwou which referred to cattle in general, irrespective of gender. Indeed it is only in Celtic and Germanic languages that 'cow' has come to refer to the female, in every other Indo-European language it refers to the species and both genders.
Boar and Sow
Swine or pigs would doubtless first come to mind, these terms are also used for bear, guinea pig, hedgehog, jellyfish, and mole. Nothing is known of the origns of boar before the Proto-Germanic bairaz, although we can be fairly sure it was created by the Germanic speakers as it is unknown outside the West Germanic-speaking regions. Sow can be traced to Proto-Indo-European su which began referring to both genders and is thought to have been imitative of the noise made by pigs. Again the use for animals other than pigs is comparatively recent.
Buck and Doe
Deer or even rabbits come instantly to mind but these terms are also used when speaking of antelope, reindeer, gerbil, goat, hamster, kangaroo and mouse. It seems none of these were the original references, indeed both words seem likely to have once referred to different animals. In the case of 'buck' this undoubtedly came from Proto-Indo-European bhugo meaning 'goat', while 'doe' is an Old English loan word and related to Old Irish dam 'ox' and Old Welsh dafad 'sheep'.
Stallion and Mare
After horse we may think of zebra, but should also consider the seahorse where the male and female are known as the seastallion and seamare. Of the two the female version is by far the oldest and originally referred to simply 'horse'. Stallion is thought to be derived from Proto-Germanic stalla or 'stalls' and where the beast would be kept in order to service the mares.
Dog and Bitch
Both terms are comparatively recent and the origins unknown. Until the 15th century both sexes were simply known as hounds.
Cock and Hen
Not only birds but also fish such as the salmon. While 'cock' is something of a mystery, 'hen' is derived from Old English hana - although hana referred to the male bird and came from the Proto-Indo-European kan meaning 'to sing', still seen in the word 'chant'.
Ram and Ewe
The origins of the male 'ram' is not clear but could refer to 'strength', the word seen in Old Norse rammr 'strong'. As with earlier examples the female 'ewe' began as referring to the animal in general and is derived from Proto-Indo-European owi meaning 'sheep'.
Gander and Goose
From the Germanic gans meaning 'goose' also comes 'gander', the root gans imitative of the bird's honking cry.
Stag and Hind
The male 'stag' has not changed since the Proto-Germanic stag with the same meaning. This comes from the Proto-Indo-European root stegh 'to prick, sting' and used in the sense of an animal in its prime. The female 'hind' is derived from Proto-Indo-European kem meaning 'hornless'.
Cob and Pen
The male swan or 'cob' may share an origin with the surname 'Cobb', in which case it simply describes a large beast. The female 'pen' shares an origin with the French penne or 'feather' in simply meaning 'to fly'.