An image of a dolphin from
Kongelige Bibliotek, Gl. kgl. S. 1633 4ยบ, Folio 60v
It is referred to in later documents as the 'mereswine', suggesting the charming idea that there may have once been a freshwater (mere) porpoise, now lost and forgotten, presumably because we've eaten them all - with frumenty. We're obliged to @HenryBluntDD for pointing out that "'mere' in Middle English could mean any body of water, from pond to ocean, fresh or salt. It originally meant "sea" and the compound "mereswine", inherited from Proto-Germanic, preserves that use. And, sadly, European River Dolphins never evolved."
Original Receipt in the 15th Century 'Austin Manuscripts' (Austin 1440)Ffirmenty wit Porpeys. Take faire almondes, and wass them clean, and bray them in a morter, and drawe them wit water through a streynour into milk, and caste it in a vesse. And then take wete, and bray it in a morter, that al the hole ho be awey, and boil it in faire water til it be wel ybroke and boyled ynowe. And then take it from the fire, and caste thereto the milk and let boil. And whan it is yboyled ynowe, and thik, caste there-to Sugur, Saffron, and salt; and then take a porpeys, and chyne him as a Samon, And set him in faire water. And whan it is ynowe, baude it, and leche it in faire peces, and serve it fort with firmanty, and cast there-on hote water in the diss.