The Beastlands is the plane of idealized nature. The prevailing theory is that it was formed by the will of the Titans, the proto-gods born of chaos, blamed for the creation of material world, as a conceptual model of the Material Plane, though this is perhaps an anthropomorphic misapprehension, attributing as it does rational, fathomable motives to alien their minds.
It's location (if a conceptual realm can truly be said to have location) between Arborea and Elysium has been ascribed to mere sympathetic aggregation (owing to all three evoking the natural world), though some have argued equally persuasively that it partakes of both the harmony of Elysium and the carnal nature of Arborea.
The Beastlands is primeval wilderness, unspoiled by the action of thinking creatures. Its inhabitants are are animals--or rather the iconic spirits of all wildlife, fierce and beautiful. These animals may speak if they wish to do so, but it is wrong to imbue them with human characteristics beyond this or processes of thought. At all times they are wild beasts, and are not given to acting outside their natural roles.
Travelers who spend time in the Beastlands will feel the call of the beast within. Lycanthropes are empowered by the realm, and other humans may be susceptible to being transformed into animalistic forms the longer they stay. The partaking of certain foodstuffs within the Beastlands hastens this transformation, and varieties of Bestland fungi are sought for ritual use on the Material Plane for their potent connection to this realm.