Neither random animals, trees or rocks are
characters, and that’s what the
solo tag is about: “image with only one character.”
Harry the bear or the butterflies Discord turned into in
The Return of Harmony? Sure, they’re characters, so Fluttershy pictures with them aren’t solo anymore. The critter of the week she’s feeding or talking to for exactly one scene in the episode, like the window dressing they are? Not so much.
An important point of tagging is to allow people to find what they’re looking for when they’re browsing the site, and I’d rather not miss pictures like
these two when looking for solo screencaps.
A picture or
two of only Rainbow Dash flying high in the sky next to some birds suddenly aren’t solo anymore, even though they’d be window dressing about as much as a night picture with the Mare in the Moon. Even though she’s the focus, maybe the point even, of the picture. What people would be looking for when browsing the site with tags.
Solo focus says “an image with multiple characters with the focus being on just one, generally in the center with the other character(s) obscured or mostly offscreen,” and is usually used on cropped screencaps and/or fanarts from what I’ve seen. Not quite the same as a full picture intended as such.
If the butterfly talks back in the picture, yeah, not solo, because they’re actually a character. If it’s just there looking pretty? Yes, solo.