Werewolf-type creature transformations - what could trigger them?

Nothing wrong with just rolling with the full moon. Unless your world happens to have more then one moon, then it could certainly twist the lycan's up a bit. Werewolves and lycanthropy has a long history and a lot of possible set offs to go into wolf mode. Perhaps the easiest way is to simply have them be able to go to wolf at will. It's as regular as they make it and could also have the unwilling beast mode that comes from too much stress, lot's of blood in the are and damn, they are huuuungry, they need to solve a mystery and the nose knows or they're just bored and want to chase some sheep.

Could also have an activating phrase for it. In which you end up with Power Ranger wolves. Or one yelling 'Transform and run out!'

 

Riffing off Devor a bit more...
A comet/asteroid, made of a particular crystal which when it is at the right point of its orbit re-emits the sun's light at the frequency to trigger the lycanthropy? Fragments of this crystal could harm or aid Lycanthropes in their everyday life?

 

I much prefer the "able to transform at will, but gets forcibly transformed under certain conditions and has a lower control over their beastly instincts during this time", but that still involves conditions. I'd say, if you don't want to use the moon, then make something else in your world happen regularly and release something that triggers the (forced) transformation. Maybe have a plant that releases pheromones every 5 weeks(you did say that the number 5 is pretty important in your world), or maybe some kind of mini, world-wide tremor that happens on a regular basis.

 

Maybe just the appearance of a particular star, staying with the celestial bodies idea? Maybe a seasonally change, or type of weather?

I like Devor's reference about pollen, maybe even the pollen count of a particular day could trigger things, though that would make it very difficult for anyone to know when the change was likely.

 

So at the moment, transformation at will is not a thing? But it could be in the future?

 

Most worlds have lycanthropy as being a Bad Thing, so you're not too unusual there.

 

Have you considered making the werewolf transformations seasonal? Just as many animals, birds etc have breeding seasons I don't see why werewolf transformations can't be seasonal.

 

Therianthropes are considered to transform only when they meet certain conditions. Lycans transform when the moon is full. But, I think therians' transformation are triggered when their animal instincts become too much for them to handle. But, animal instincts often reduce when a wild animal is tamed and domisticated, and then trained to overule this instinctual side. So, I think your MC gets his wild side tamed by from an event. And then tries to activate his beast form through training how to trigger a beastial instinct. I think you can make territorial instinct as a trigger, also hunger for prey, and the drive to protect a person close to him. Well the trigger would depend on what type of animal the therian is affiliated to. In the case of wolves, they are territorial, they hunt in packs, and follows the will of the alpha.

 

So most werewolves or werebeasts in this case have one of three methods that causes a transformation.

Unwilling-Classic full moon type werebeasts trasform on the full moon whether they want to or not an often have no control or depending on the story aybe they do.

Willing- The werebeast shifts at will a great example is 'grimm' which has humans that turn into animals when they want to or are under some sort of enviromental stressor. This is becoming an increasingly common type although should not be consitered a 'werebeast' as much as a changeling or a shifter as the very nature of werebeasts is that they can not control their animal half.

Urge-Now this is one I went with and that I do not see too much. This is the idea that the animal half of the werebeast function on it's animalistic instinct and can overpower the human half under the correct conditions, very simalar to 'venom' or 'tokyo ghoul'. For example, that whole werewolf full moon mess stems from the fact deer and other prey are infact active at dawn, dusk, and night time hours and the brighter the moon the more likely they are to be out. As such, wolves hunt more on full mons cause more prey is out and they have better night sight than deer. in older times people knew this so the believe was the wolf half of a werewolf comes out to hunt as well on a full moon. However they could have all sorts of species specific triggers a werecat for example maybe if someone or thing is running away or running near the human form as cats have a very strong chase instinct. This type may also be enticed into shifting if it is in the wild and the human half is dying of hunger and dehydration or being attacked by another animal as the animal half often understands it needs to protect the human half or both of them die. It is up to you whether these two halves can communicate with each other at all and how intelligent their animal half is.