Animus
Basic Information
Anatomy
When it is first awoken, an animus has no form or substance, merely a will which drives its spiritual force. It remains subject to the beliefs and attentions of mortal(s) throughout its existence, but this is especially true in its early days and for the individual mortal(s) that caused it to split from the World Spirit in the first place, and this can cause it to take a form based on those beliefs and attentions. The example most often given of this effect is the propensity for river anima to have serpentine forms due to mortal perception making comparisons between the shapes of such creatures and most river formations. It has been discovered that this effect can be intentionally controlled when the mortal(s) involved direct themselves with intention. Should an animus be awoken intentionally by a single mortal who chooses to think of it as an individual of its own species, the resulting animus will nearly always take a form somewhere on a spectrum between whatever part of nature birthed it and that species. Where it falls along this spectrum depends on the skill and imagination of the mortal, and on their own ideas on the matter.
Genetics and Reproduction
Anima do not reproduce, more can only be awoken by the interaction between a previously dormant aspect of the World Spirit and a mortal seeing it for the first time. Though it is possible for smaller anima to awaken in the heart of the known world as new plants grow, etc. any newly awoken large anima like those of rivers forests or seas can only come into being far from civilization's borders as explorers first set eyes on new frontiers.
Growth Rate & Stages
An individual anima may grow based on two entirely unrelated axes: the size and age of its host, and the beliefs of the mortals it interacts with. As a tree grows, so does its animus, but even if a young tree stops growing, its animus may grow if the tree or the animus itself is considered sacred or important to nearby mortals. The often cited example of this second phenomenon is the animus of a bonsai sapling praised by the divine emperor of the Rimpan state immediately growing in size and power to a size rivalling that of the nearby forest due to the reverence it received from the country's entire population.
Ecology and Habitats
Anima are not utterly restricted to remain near their host nor even in areas with similar circumstances, but their strength and activity tends to drop off precipitously when they get further away from each.
Behaviour
Apart from supporting and nurturing the natural processes of its host, an animus' behavior is entirely determined by the mortals it interacts with. There are even examples of particularly skilled and determined mortals changing an animus' driving purpose to be entirely counter to the welfare of its host, like a tree animus blocking rain and sunlight from passing the crown of its host. This is used to great effect in many sacred places in Kanō Rimpa, protecting an outdoor space from the elements so long as the nearby people can tend to the needs of the tree as the rain once would have.
Additional Information
Perception and Sensory Capabilities
Every anima has a perfect understanding and moment-to-moment knowledge of their host; no matter where a tree animus may go it remains perfectly connected to and aware of its tree, etc. Beyond this, any additional perceptions and senses depend almost entirely on the initial beliefs and/or goals of those who caused them to awaken or to a lesser extent an aggregate of the expectations of the mortals it has met since.
Lifespan
Up to eternity, depending upon their host
Geographic Distribution
This caught my eye at first because it reminded me of a creature I created called the Amicus Imaginarium, and then your article just kept getting more interesting from there. I love the concept, and I love how you flesh it out over the entirely of the piece.