When it comes to Affinities, Beast Type is by far the most predictable. Assuming you know what caused The Discovery in the first place, that is. These Knights discover their Affinities through the great outdoors. Beasts, animals, plants, or environments in general all fall under this type.
Knights with Beast Type Affinities evolve to mirror their discovery. This evolution includes most aspects like anatomy, behaviors, and/or diet, but within reason. They're still humanoid first and foremost, so rest assured that Beast Types won't become feral or root themselves into the ground.
A Mouse Knight won't shrink, a Cat Knight won't try to hunt the Mouse Knight, a Dolphin Knight won't instinctively commit vile atrocities, and a Hamster Knight won't try to brutally kill itself at seemingly every opportunity. But at the same time these are all still regular people so they very well may try to do all that out of "free will" or something like that.
Aside from the convenience of 3 categories that sum up to "Profession", "World", and "Magic", the reason plants and environments fall under "Beast" is more semantic than anything. A Tree Knight is closer to being an Treant than a regular tree. Even if they have the ability to photosynthesize or a lack of actual humanoid organs, the fact that they're a hybrid is enough to justify their inclusion in the type. But this is also why sub-types exist.
Awesome! \o/
Thank you!