dba_afish said:
the first post to have the tag was this:
post #137823
from the comments I gather it was a character from a now-lost CYOA blog, the little robot pony was presumably called Hugbox, the other then being Heart Fixer.

Now I'm curious as to why we call hugbox hugbox. Where did "box" suffix come from?

YummyTummySketties said:
Now I'm curious as to why we call hugbox hugbox. Where did "box" suffix come from?

If you mean in the specific context of fluffy pony, I'm not sure if there is added context there.

In general usage, hugbox might be named after the actual device, definition 1 here
(which I think originated from an autistic person observing that being in a cattle-crush helped calm them down.)

it came into the fluffy pony community the same way "plot" and the like became MLP slang: 4chan.

hugbox was initially slang used derogatorily in much the same way as "safe space" and "echo chamber". essentially it was a term used to refer to communities who were seen as, wholey/aggressively "anti-negativeity".

not sure exactly how it came to be used as the cuter/happier genre of fluffy pony; it's pretty much unrecognizable...
I could see someone misinterpreting it to just mean "happy stuff" in general, later using it when talking about some fluffy content at which point it just stuck. there was a lot of board-hopping that when the community largely resided on 4chan. mods on /mlp/ hated fluffy pony, and mods elsewhere hated everything related to MLP (fluffy pony included). perfect environment for picking up slang you don't understand and misusing it.

and then "-box" eventually just became a suffix for genres of fluffy pony content.