Recently I made a backstory for Enzy as guardian spirit trapped inside the plush version of himself. And I also added him an ability to transform back into true form by will. Yet I was wondering what other triggers can exist to begin the transformation like emotions, items or precise body point?
Would like to hear your ideas and thoughts about it :3
There could be several triggers possible associated with each of the 5 senses. Touching a certain part of the body (like a nose boop), hearing a certain word (like someone saying āBiggerā), breathing or swallowing something (incense, potion, oxygen, etc.) or simply seeing something in particular (like seeing a pump or something).
Alternatively you can go with most of the emotions wheel (anger, sadness, fear (pufferfish)).
I have a silly fasinations with werebeasts, and often worldbuild around the concept.
One of the things I like the most is thing about how they would turn: turning during a trigger would allow them control over their beast form, whereas doing so in the opposite they would become feral:
Canines following the standard moon cycle: full moon granting control, whereas turning during a new moon would make them a danger puppy. Felines following a similar one but inverse, where they have control during the new moon. Bears would follow the seasons, the closer the winter is the more difficult it is to control their beast form, and their hunger. Raptor birds would follow atmospheric pressure inverse to sea mammals. Reptiles and mustelids their temperature, so on and so forth.
I really liked the idea of natural cycles being integrated into the concept of changing and transformation, made it feel āorganicā. Play around with the idea of an especific location, biological state (like the Hulk), state of mind, etc. Thereās plenty of inspiration to go around.
Cannon for my sona- when I get flustered or humiliated my antlers get warm and tingly and it causes me to shrink. 
I had an entire class of āBeast Knightsā who have to FIND their triggers. The main character was the fourth son of a noble house, disowned by his father (but not his brothers) due to misconceptions about the Knights.
His trigger is actually being motivated to do something for reasons OTHER than honour and his family name.
Another character canāt change without the sight of his own blood, and another became one late in life and triggers with memories of his late wife.
Donāt start me on the other TF triggers Iāve made like Pokelite and such.
Blood moon is indeed awakens something dormant inside your body
