Do Umbral Aniimo spawn in the open world?

Random means you cannot plan around it. There is no fixed spawn point to camp and no condition you can set up to force one. They turn up occasionally, wandering, so the only method that works is time spent outdoors and the habit of actually looking at what is moving in the distance instead of riding past it.

Because the encounter is uncommon, what you do in the moment matters more than usual. Before you register or release anything at camp, check its Alpha status, whether it is carrying a weather or special form, and its potential grade. Catching one copy is also not the end of it. Better potential, different traits, sparkling versions, alternate forms and Prismana forms are all separate collection goals, so a second sighting is still worth stopping for even after the entry is filled.

Wild ecosystem surges

Random wandering is not the only window. Umbral Aniimo also appear during wild ecosystem surges, which gives you something closer to a cue. If a surge is running in a zone, that is the time to be out in it rather than sorting your camp.

Dark forms do not come from the world

Do not confuse these with the dark forms. Darklers and the Umbrabow form are obtained exclusively through Egg Heist, and no amount of roaming will produce one. Egg Heist currency accumulates as you play it, and you can exchange that currency for an Umbrabow egg directly, which is the dependable route if a specific dark form is what you are after.

Side quests are worth keeping on the list too, since certain ones unlock alternate forms and secret evolutions.

Not settled: Dark forms being locked to Egg Heist is the current state rather than a permanent rule: the developers have confirmed that unique Aniimo forms introduced in the battle pass will later become obtainable in-game.