There are five of them: red, white, yellow, pink, and green. They will follow you silently and faithfully and cannot be interacted with or spoken to.
Locations:
Newest versions: in the puddle of water near the waterfall just outside Whiterun Stables near the Khajiit Caravan camping spot.
Old versions: they are located at the copse of trees further away from Whiterun, see images for map location. Note: In the old versions they either behave like Orc NPC followers, or like pet dogs. I wasn't satisfied with that implementation so in the newer versions they are eternal silent companions who don't speak.
Textures:
They will use whatever mushroom retexture you are using. I use the ‘realistic mushrooms’ in the screenshots by me. It was in Skyrim Ultra HD by MysteryModder, but it’s not on the Nexus anymore. Any mushroom retexture will look great, something like renthal's flora for example.
Dragonborn DLC required for one of the mushrooms, the white one, without it he will be purple with missing textures. Dawnguard DLC for latest versions for pretty magical effect shaders.