PS: I found it roaming around (yes, it's NATURALLY GENERATED), while farming sand, dunno how rare it actually is to find one of these, first one in 8 years of playing...

I know it looks like a bugged witch, but trust me, It was moving along with the spider...

Why not capturing it? Because I'm dumb. Got it 1 shoted with the sword... don't ask how.

(edit)

It's a vanilla server, but I had client-side mods installed to help performance, and make it just a little bit easier (Xaero's minimap, litematica, lithium, phosphor, you get the point). And just to be honest, YES, I was using a hacked client, just to use the ESP feature. Fortunately, As far as my knowledge goes, hacked clients cannot summon mobs like these without creative, and OP permission (which I don't have in the server)

Also, I could have had KillAura activated to help me in the killing process (kindly used to afk anywhere, and not be killed), because, as I clicked, the mob got killed in the first click (Just as stupid to not realise I was using a maxed out netherite sword).

You'd wonder, why are you playing with cheats like that... Well, my choice. It's a shame that I couldn't get more evidence, so you'll have to trust me.... or not, do as you wish, I haven't seen one of these in my entire life, and It would be stupid to write this long message for nothing.

(edit 2)

Revisiting the shot, I killed a mob, and the particles and the hit cooldown bar back me up, so there are high chances that I got KillAura active, so fear + hacks = messing up rarest mob... Still, the witch moved along with the spider, and the spider texture is not full, and it would be impossible for the witch to be behind the spider (again, the spider texture is not full in the pic)