TAKE THE 541 LORE BECAUSE RP GAVE ME MOTIVATION

This is actually part of the thing in the Showtime animatic I did a a while back (or the aftermath of it)

Here's the without words version

Anyway TIME FOR OPEN SPECIES >:3

Yes, I'm opening 541's species. They're called Zinoids! (A mix of two names that were suggested by Squishy and Quicklight)
This picture specifically is of 541/Xercantoli (that's how he was known at the time) and his adopted son, Zeraphyn, or Zera.

Anyway moving on

Zinoids are an entirely masculine (male) species, so therefore they are asexual (meaning they can reproduce without a mate). They all have long names like the ones above lol. They are incredibly flexible, since their bones are mostly made of a fleshy material (although it's a mix of that and actual bone). This is how in the prologue of TI, if you remember, 541 turned his head entirely upside down TwT

The slime/goo/whatever you wanna call it that covers their entire body can be any color, just a dark hue of that color. Like if it's green, it has to be a dark green, not a neon green or anything like that. They all have the chest mouth, and some actually have two tongues in the chest mouth (it's rare but it happens \_[:b]_/).

They can be as old as a thousand years or more, and they start growing their tentacles at around 50 y/o (Zera is about 30, which is why he has none), and they can grow 2-6 tentacles throughout their lifetime. They can also forcefully form more, but it takes away from their height and size, as well as drains energy because it's harder to force new tentacles compared to letting them grow. Their normal height can range from 3 feet tall to 7 feet or higher

So the eyes. They're actually normal eyes, but only the pupils are visible because the scleras are the same color as the goo substance. They usually glow, but it's not uncommon that they don't. Sometimes the color says something about the Zinoid's personality (although I made 541's eyes red for a spooky look, his eye color actually doesn't say anything special about him). Other than that it can be a bright version of the goo color or it's complement (For example, I have another Zinoid boi that is dark purple with yellow eyes, while Zera is a dark gray-blue with light blue eyes).

Their blood is very dark as well. Its just like human blood but it has different qualities and can be almost black.

Now for some Zinoid lore.

541 is currently the last known Zinoid. They were killed off by humans (dang we suck sometimes) for multiple reasons. Them being a bit unsettling in appearance, it kind of became a stereotypical thing that they were dangerous. So humans being humans, we killed them all off. Another reason was the qualities mentioned of their blood. It can have a kind of acidic quality to it when exposed to certain things (such as human skin, tissue, and certain metals. Which is why 541's possession victims were always incredibly gory) which can make it valuable to humans.

About the possession thing, too. They can all do that. It can only happen if the Zinoid is able to enter the host's bloodstream (like through a cut). They can do it whether they're alive or dead, but each can turn out differently .

If the Zinoid is alive and possessing someone, then the host will be stuck in their own subconscious when subject to the Zinoid's control. It can take a lot of strength, so there are times when the Zinoid will give the host back control to regain their energy. The exposure to Zinoid blood will, however, eat away at the human's skin and internal systems until the host is entirely gone and the Zinoid is once again separated.

If a dead Zinoid is possessing someone (as in, the Zinoid died but managed to make contact with a rupture in the host's external skin and get into the bloodstream) it's only slightly different. The Zinoid will have a stronger control over the host, since they can do more and be forced to rest much less than a living Zinoid. The blood will still eat away at the host, but instead of the host dying completely, the host will then become the Zinoid. Basically, if the Zinoid is in there long enough, their instincts, behavior, and thoughts will completely override the host's, the host's soul will either die or be forced out of the body, and they will slowly evolve back into the Zinoid that possessed them. So basically they use the host to form a new body.

And uh.... I think that's it. Goddamn that was a lot.

Again, this species is open and you can make one if you want >:3

Edit: Oh, I forgot to mention this. In TI, 541's voice sounded very gargled and raspy. That's because he'd been in his containment at the SCP Foundation without fresh air for a long time. Deficiency of true outside air can cause their voice to clog up like that.