Mythology? .... maybe

Pretty much any mythology with an interactive Pantheon does this (the Olympians could be seen/not seen solid/ethereal when interacting with humans).

Fae myths also account for corporeal and non-corporeal beings.

Spirit guides, such as animal spirits in Shamanic belief systems can be anything from astral projections of an animal, anthropomorphic versions of an animal, or a physical animal that leads the human on their quest.

I've heard the term Metaphor and in my story ESSENCE used the term phase shifter for a creature that, at will, could change between solid, gas, liquid, and plasma (the four main phases of matter). There is a fifth specialize phase at near absolute zero but it irrelevant to a mobile creature in a SF.

MarieSalvros said:

Is there anything in mythology, or anywhere else for that matter, where there's a species of people/animals/things that phase in and out between solid and not-solid? I'm looking for a name to call this group in my WIP and I kind of want it to be connected to something referenced in history.

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Non-corporeal myths are everywhere.

Greek/Roman: Spirits or ghosts.
Celtic: Kelpies, naiads and dryads all are insubstantial when they leave their body of water or home tree.
Germanic: Poltergeists, and vampires could turn into mist.
Arabic: Djinni could turn into clouds of smoke, and their ghouls ('ghul') could turn into green mist.
Japanese: Oni -- kind of a ghost, kind of a demon.
Philippine (one of my favorite pantheon of myths): Dila -- a tongue that passes through walls to assault people inside houses, and the ghosts called Multo.

Where your story is based matters a lot. You might be best off making up your own name for the creatures, one that fits in with your world and the characters in it.

Kalayna Price, in her Alex Craft series (link to her site), calls people who can move between planes of reality "planeweavers"

I suggest making something up too.

Joseph Campbell Hero of a Thousand Faces says that a heroic story is a descent into the world of the fantastic...all heroic stories are a transition into and back from the spiritual world. An example that may seem a stretch at first is Rip Van Winkle.

He basically sleeps through his spiritual journey and when he returns he immediately has the power to look on the regular world with the insight of a spiritual traveler (forty years have passed).

Elementals; those nebulous entities whose natures mimic the traditional elements of alchemy - Air, Earth, Fire, and Water.

References can be found in the folklores of many cultures.

http://www.themystica.com/mystica/articles/e/elementals.html