This find has quite a history, as is explained in the video.
Miyazaki himself presented the first footage of the Chalice Dungeons at the first ever PlayStation Experience Convention in December 2014. Showcased in the footage was most prominently an Undead Giant with two blades for arms, one of which is on fire, enabling him to set the area around him as well as his victims aflame.
Bloodborne then released in March 2015, but the Flaming Undead Giant was nowhere to be found in the dungeons. Along with other things such as Boss Rush and the Revered Great One's Coldblood, this version of the Undead Giant was generally declared as "Unused Content", that is, not having made the final cut of the game.
Three years later, on 19th November 2017, a group of Tomb Prospectors, led by KolbrotKommander (the creator of the dungeon) made the impossible discovery by officially sighting the first ever Flaming Undead Giant in a Depth 4 Cursed and Defiled Root Chalice. And so it goes full circle.