This one leaves things open for all sorts of possibly nasty comments.
(redakto: post pli ol unu jaro… I was right. )
I believe it's because of the Norman conquest, when huge amounts of French words flooded the language. The words for the animals remained Anglo-Saxon (because Old English speakers did most of the raising of animals), and the words for the meats and the dishes became French/Latinate (because the Normans did most of the eating and the cooking!)
And something to add: While this examples are rather distingushed, there are many languages where pork is made from a pig by using affixes. This is exactly what you see here (making pork from pig with suffix - ijx-). It is a common way to make it in a synthetic languages, and Esperanto is just one of them.