Not seeing Mr.Seburoh's post, so I'll try to take over on the lore for today
The Crystal Lizard
Pretty sure Seburoh covered this ages ago, but just as a recap, these big little guys are what you get when those fearful little Titanite Lizards somehow manage to consume some humanity (Humanity: Not even once). This isn't too surprising, as even humans who consume excess humanity tend to turn into monsters.
The Black Knight
Hoo boy these guys have a history. Basically, waaay back in Dark Souls 1, during the first age of fire, the Witch of Izalith attempted to recreate the first flame, however she failed, and her creation warped and consumed her and her daughters, this was the Chaos Flame. From this flame, all Demons were born.
The demons were quite hostile, and the Silver Knights of Anor Londo would constantly struggle to keep them contained, many of these knights, as a result of the Demon's fiery powers, would have their armor charred black.
Some time after this, when Lord Gwyn set out to rekindle the First Flame with his own soul, these Black Knights would travel him, making pilgrimage to the Kiln of the First Flame. The attempt to rekindle the fire created so much heat that all that would remain of these knights was their charred armor, filled with Ash.
Taken from the description of the helm it was so kind to drop for him:
The knights served the First Lord Gwyn, and followed him into the flame upon its linking. They became ash, but still wander the realms to this day.
Skelly-face men. a.k.a. Dickwraiths
These guys are another remnant from the original Dark Souls, former members of the Darkwraith covenant, a group of Humans from the old city of New Londo, who were tricked (or not) into consuming the humanity of other humans by the Primordial Serpent Kaathe, who wished to put an end to the age of fire, and usher in the age of Dark.
Kaathe wanted the Darkwraiths to consume other humans in order to become the Lord of Dark, who would be able to defeat the gods and snuff out the first flame. To keep it short, The Darkwraiths wanted to bring about the age of dark, and turned to the powers of the Abyss in order to do it.
As is noted in the description of the Darkwraith item, The Dark Hand
Weapon that allows its wielder to evoke an art unique to Londor, the land of the Hollow. It is also said to be an ancient relic of a Primordial Serpent. Well, there's that pesky primordial serpent I was talking about, but... Londor? Didn't I say these guys came from somewhere called New Londo? Well... yea. It's believed that over time, New Londo become what would be Londor, and interestingly, we've already met someone hailing from that land, though you may have forgotten about him.
Waaay back when we first came to the undead settlement, we took a friendly chap named Yoel into our service, well, he claims to be a pilgrim originating from Londor, he actually has quite a storyline attached to him, but they aren't really getting into it this playthrough, and discussing it kinda spoils the ending, so I'll leave it there.