In D&D, fungi are considered plants, which means a fairy circle is a valid target for Transport via Plants.
For anyone wondering: fungi are neither plants nor flesh, they are their own thing. Only learned this while learning the farmer job.
And in IRL taxonomy, they’re more closely related to animals than plants, but probably diverged long before sponges came about, let alone other animals.
Yeah, there's a tree stride variant in Out of the Abyss if you agree to become infected by Zuggtmoy (fungus queen). Basically it's Mushroom Stride. :)
See, I was always taught the mushrooms were just growing afterwards, wherever a large amount of magic had been discharged (like when a portal happened). They were growing after the fact, soaking up any residual magic splash.