Aesop

A commission for a GURPS-type setting.

In this world, magically active patches of ground later called "Aesop Domains" started to appear in the early 15th century, France being probably the first place affected. Any mammal spending a few minutes in a Domain turns into (within a few hours) a roughly human-sized and humanoid bipedal sapient - a Furry, if you will, locally called "Aesops." Although they can breed with their own kind, restraining them from breeding does not stop the number of Aesops from increasing - wherever an Aesop dies, another Domain comes into existence, and Domains cannot be eliminated once they come into existence. (The first Domain was of unknown origin - Alien Space Bats? A Wizard Did It? Who knows? - but all later ones were due to Aesop deaths.)

So, the world has had to adjust to an ever-increasing number of non-human intelligences sharing the planet with humans. This has led to not so much butterflies as Rocs, and it's surprising the world looks like OTL as much as it does. The Protestant Reformation was complicated by the arguments as to whether Aesops had souls, were just smart beasts, or were creations of the devil, which added new and exciting doctrinal disputes to have wars over. The Papacy split into Pope and anti-Pope again (for a while there were three) and hijinks ensued. 

As Aesops increased in numbers, they largely replaced humans as slaves, and by modern times much of the world's labor was done by Aesops, with of course consequences in the form of revolutions, Aesop massacres, and such oddities as socialized slave ownership. On the positive side, human racism, faced with a truly other other, receded, and inter-human wars became somewhat rarer.

As of 1994, there hasn't been a truly major war in over a century (the Dozen Wars of the period 1932-1948 were something of a damp squib, killing under a million people total), but the situation is becoming critical. The Aesop population is now matching that of humans, and global food supplies are increasingly stretched, and new Aesops show up every time a mouse runs over a Domain. Technology is little more advanced than our world's WWI era, greater religious violence and an increasing over-abundance of labor having slowed scientific progress. (Supernatural thinking is a lot more prevalent even in developed countries, since Aesop Domains and Aesop transformations from animal to Furry form have nothing to do with physics and biology as they are known). Many countries which use a lot of Aesop labor are trying to wean themselves off it, while countries with fewer numbers of Aesops are talking about eliminating them from human society entirely: outright genocide is still frowned on, but there is a lot of heated talk, especially in the Aesop-crowded regions of western Eurasia. 

Africa, with little foreign need for human slave labor (and therefore no Slave Trade issue) and an even harsher disease regime, has seen little outside penetration of the interior, with the Arabs (who are Very Determined to spread proper Islamic orthodoxy) being the main exception, and even they mainly work through the more disease-resistant black Africans and racially mixed coastal Swahili. This may change, as genuinely effective tropical medicine is finally being developed. 

Brazil, the world's most advanced nation, hopes to use Science to control the Aesop population non-violently and create a state of peaceful co-existence (well, one where Aesops accept humanity's benificent leadership. They're not perfect goodies). Their north American rival, the Avalon Confederation, sees this as a "betrayal of mankind" which will lead to all sorts of perverted terrors and armies of Aesops eventually marching north: the current Avalonian president, who has promised taking a "stand for humanity" and "against the growing Aesop threat", is making a great deal of propaganda hay. 

This will probably not end well.