I was on my way  back from fixing a transformer after a power outage and i came across this thing, a weird black box that started to chase me when i first found it, but after a while it turned docile and started wandering aimlessly? this thing even showed on top of interfaces including the pause menu, it crashed my game when i decided to touch it
this thing looks more convincing in the recording i got of it but i don't know how i would upload it here

This happened to me as well. At first I thought it was one of the treehouse Arirals messing with me and her model or cloak had just bugged out. But nah this dude just BOOKS it to you and starts just hanging out. He's cool, but once I reloaded my save it was gone

This little fella visits me sometimes, too.  First encounter I did not notice him in the server room and I ran into him and the game crashed.   The second and third time I encountered him, I saw him approaching from the corner of my eye and made sure to avoid him.  When moving from room to room, I did not turn my back on him, because if I did my screen would flash black.  So, I would strafe and backpedal while I did random chores and eventually, it bee-booped out of the room and disappeared.  

i encountered it again, but this time instead of wandering around it's just standing still, it backs away when i try to get near it, it follows me too when i walk away, it's just keeping a certain distance from me

it seems to change between different states, now it's doing nothing but walking away from me, the box changing in width in different ways as well
...and it just disappeared, weird

NEW HEROBRINE THATS REAL!!!!

[|87 saw this dude, good thing i saw this post beforehand, but i was sleeping when it spawned and i saw a time lapse of it running around in front of the base

yeah i consistently got this guy to spawn when i played for a long period of time, he kinda just wanders around near you and backs up when you get too close

Happened to me while the bad sun is happening (it fucking fits so well!) sadly they left

seems that the librarian got it too in his recent video, must be very common for it to show up huh

This happened to me too, but I can't say that I played for long at that time.

https://itch.io/t/4007021/bug-report-spoiler-alert-fatal-error-

sounds like an average american encounter

i just  got it to appear right now, but strangely when i look at certain angles now my screen turns completely black, the black box has stood still and not moved at all during this
i've recorded this too but once again i'm not sure how i'd share it
edit: the black blox started moving and ran away and left, my screen stopped blacking out when it started moving

I encountered this guy a little after 1:30 am IRL, I dont think I had been playing that long maybe an hour or two. I assumed it was a bugged out kerfur because of the way it sun in a circle in my garage. Until I realized kerfur was standing right next to me. Its behavior was a bit different from what you described. It went around my base checking each door before coming to the bedroom (where I decided to hide with my emotional support kerfur.)

As detailed in the SS, I could pause, but the box would overlay the pause menu. I was also able to sleep and watched it run around my base before standing on my porch waiting.

It seemed like it went to every door in my base with a keypad, but more so just felt like I was being observed.

10/10 do recommend, scared the pants off me. 

this is anomaly