I came across a place in there with a black and red ball, but when I started getting close, it vanished, just like the Crystal Lizards (except it was black and red, not blue and white).
I went back to the bonfire, rested and went back to the same place, nothing was there.
I kept farming the slugs until I found that thing again, so I decided to stay far and get a screenshot, as I couldn't find anything about it on the wiki:
After taking the screenshot I decided to try and kill it... Since everytime I moved close to it it disappeared (even though I was running at lightweight and wearing the rusted iron ring) I shot a heavy arrow in it that inflicted 4 damage. The creature disappeared again.
Does anyone know what is that?