What causes an Aniimo's form to change?

In practice this means the same species is not a fixed picture. Catch one in another region and the Aniimo you end up looking at can be a different thing on screen. Weather and the in-game clock do the same job without you going anywhere. Stand still long enough and the conditions around you change, and so does what turns up in them.

The important knock-on is that these same conditions also govern spawns. Habitat, current weather, and time of day decide which Aniimo appear at all, and location, weather, other environmental conditions, and the day-night cycle together shape the form they appear in. So where you hunt and when you hunt is one decision, not two. Picking a rainy night in a particular habitat is picking both the encounter table and the look of what is on it.

Hunting a form you actually want

If a form is tied to a region, you travel for it. If it is tied to weather or the clock, you wait for it, or come back at the right hour. The conditions are read where the Aniimo shows up, which makes this a catching problem rather than a training one.

Prismana are not an environmental form

A Prismana is an exceptionally rare radiant variant of a Nova-stage Aniimo of the same species, with a modified 3D model and an alternate color palette. Weather and the time of day do not hand you one. How deep the change goes is unsettled: it is described both as a purely visual variant and as an empowered form that differs in more than appearance. Treat a Prismana as a separate category of rare, not as a product of the region you happen to be standing in.

Not settled: The environmental forms are described in terms of visual appearance, and whether one carries any effect beyond how the Aniimo looks is not established. The conditions that produce a given form are closed-beta behaviour and can differ at launch.