Here's a somewhat blurry sample of a couple of the beasts that will appear in After the Great Beast. You might notice some interesting headings in their stat blocks; Tainted, Defiled, and Vitriolic.

The Corruption has worked its way into the hearts of many of the creatures in the Great Forest. Mostly docile animals like felhorns and hawkritches are driven to aggression when this happens, often gaining stronger abilities and a mutated appearance and size. Any creature is susceptible to the Corruption (maybe even humans!), and there will be a handful of sample beasts, along with rules on creating your own!