Does the first phase of Midra count? That’s one of my all time favourite bosses (though definitely because of phase two, which at that point isn’t really a horrible little man but it counts).

Such an interesting question to which I have no satisfying answer, I'm afraid. Except for Onze, whom I did fight and enjoyed from the little time I've spent with SotE, I don't think there's too many bosses I've fought that fit the criteria well. The closest in vibes would be the Mad Clown Puppet from Lies of P because it's such a prick. Just look at this fucker:

It's not even small, actually it's bigger than P, but it kinda fits the vibe. Once you get the hang of the parry timings, the fight gets substantially easier.

The only other enemy I could think of are the Assassins or "Rats" from Sekiro:

They come in a couple variants and some have steel-enforced straw hats that deflect basically any attack you throw at them. You need the Axe Prosthetic to cleave it apart and be able to damage them. Their kill blows are pretty metal tho: They jump on your back and bend you backwards until your spine snaps. Sounds quite gruesome now that I type that out :s

I wonder a boss fight against one of these would have looked like

Trick question, I am the horrible little man

So I guess probably the Mimic Tear?

The yedi master Demi human from elden ring (not sure if DLC)

You already picked Onze who jumped to mind as soon as I read the title 👌

How about Ancient Dragon Man or Eleonora?

The player character!

• Little? Absolutely, almost everything humanoid or not is bigger than them.
• Horrible? You are regularly committing war crimes. So definitely.
• Man? Well, sometimes… Maybe needs a coffin or a mirror.
• Boss? When I'm going in a Boss arena with 80 Int. Yeah, in that case the player character is definitely the Boss.