Tooth and Claw
Tooth and Claw is a gorgeous world of unspoilt wilderness and simple creatures living simple, if often brutal, lives. It also actively reshapes any new arrivals to suit. Regardless of who, or what, a visitor was when they stepped through a portal, when they step through the other side, it is in the form of a mundane animal, albeit one that can reason and speak.
Magic does not work there, and even quite simple technologies (Pulleys, screws) seem oddly recalcient, on the rare occasions that a traveler manages to construct one without aid of proper fingers.
Climate is largely similar to modern Earth, as are the local creatures, although they do not map perfectly to real historical ecosystems, and a number of more recently extinct species' are present.
Portals to and from Tooth and Claw generally swing only one way:
Those leading to the world are invisible and sporadic, appearing in places of quiet wilderness, no matter how small.
Those leaving are more fixed, and rather more noticable.