Aristotle [ca. 350 BCE] (De animalibus, Book 1, 14.5; Book 2 chapter 12.2): [Book 1, 14.5] ...on the right side is the liver, on the left the spleen, alike in all animals which are furnished with these parts in their natural form and without monstrosity, for already there has been observed an altered order in some quadrupeds. [Book 2 12.2] In all animals the spleen is naturally situated on the left side. The case has occurred that an animal having been opened, has been observed to have the spleen on the right side and the liver on the left, but such appearances are considered ominous. - [Cresswell translation, 1887]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.29): Kylion is a marine animal quite remarkable, as Aristotle says, in which nature is believed either to have erred, or to have changed its order by its use. But it is far from believable that nature has erred: for it has arranged all things well and created for each thing that which is right and just. Indeed, with all the living creatures of the earth, both small and great, he put the liver on the right, and the spleen on the left, but in the kilion he put the spleen on the right, and the liver on the left. - [Badke translation/paraphrase]