A special rite of passage

I recently posted this on a club, but adding it for private RP only people as well.

A story in a fantasy town with a population with an unusual coming of age ceremony.  They take a drug and meditate.  In the process, transforming into something other than human (and not anything 'almost human', like elves dwarves etc).  They do not choose what they will become, it is partially based on their inner nature, and they believe, the choice of higher powers.

After transforming they are to live as the creature for a time.  Most importantly, find an opposite gender version of the creature, successfully mate with it.  The women are expect to have a child/children as the creature and raise it to whatever degree the creature normally would.  Men are expected to be involved to whatever degree males of that creature are expected to be.  Some just mate and leave (or mate with several), others help protect/raise their non-human children until they are old enough to protect themselves.

Eventually they become human again.  Return to the town, find a human spouse.  Often they choose a woman who was the same sort of creature, though it is not required.  A few times a year, on holidays, the adults of the redo the experience.  Transforming again, either returning to the wild or seeking a another transformed person.  They consider it a holy act, giving back to nature, breeding more of what they hunt or otherwise kill.

The creatures could be ordinary or more fantasy based.  Not looking to play every moment of the transformation.  But a good mix of human and non-human lives, sex and story.

Character age is flexible.  It is coming of age ritual, but it is up to the individual to judge when they feel they are ready, knowing the danger and feeling they can succeed (depending on the sort of creature, they likely have to worry about predators or rival creatures.  They also only succeed if they successfully mate, and have children as whatever they are.  Successfully raising the children is expected, though it is not held against them if they failed, as long as they seriously tried their best).