One time when I was playing Bedrock Edition, I found a weird mod called Elder Guardian Ghost that can only be summoned by commands. I know it's a hostile mob because when I tried to spawn it using commands on Peaceful Mode, it couldn't spawn. I guessed it would spawn once a Elder Guardian died, because the mob is called Elder Guardian Ghost. I spawn a Elder Guardian, and killed it, and then git the items it dropped, and then used /kill, and it didn't say that a Elder Guardian Ghost died, mainly because a Elder Guardian Ghost didn't spawn when the Elder Guardian died. Also when you use /kill in a world with a Elder Guardian Ghost, the game refers it to as "entity.elder_guardian_ghost.name", in which is weird. Also I can't interact with the mob, and it can't interact with me. The only way I can interact with the Elder Guardian Ghost is by using /kill, which makes sense why that is the only way I can interact with it. I have three theories about this mob. Theory one: it is a mob for a future update. Theory two: it is a mob used to test for a future combat update, in which is proven by there being combat test, like 1.14.3 - Combat Test, or Combat Test 8c, in which I went to the Minecraft Wiki to list the two version, here is the link:

https://minecraft.fandom.com/wiki/Missing_launcher_versions#1.18

Theory three: this mob is a long lost mob.

What do you guys think? Leave your thoughts in the comments below!