Ignatius Knight was a human man who is now a ghost.
Mine official Ignatius Knight was one of the many hopeful suitors of beauteous Melanie Ravenswood in the Ravenswoods' golden age. Like Melanie's other suitors, Knight died in mysterious circumstances which had actually been engineered by Melanie's father Henry — in his case, in the explosion of a barrel of TNT inside the mine. What was left of Knight was buried alongside Melanie's other three early suitors (Rowan D. Falls, Sawyer Bottom and Barry Claude) in a crypt at the edge of the Ravenswood estate, where the unfortunate four's ghosts still dwell, hoping Melanie will marry them even now. In the 1880's, the Phantom replaced the four Stretching Portraits of Melanie by herself he had in his Stretching Room with Stretching Portraits of the four ill-fated suitors with Melanie, from which Melanie's image disappeared as the portraits stretched.
Ignatius Knight was added to the Phantom Manor ride as part of its 2019 updates. Like those of the other three suitors added in said refurbishment, his name is a pun mirroring his way of death, as "Igg Knight" sounds like the verb "ignite" (the action that led to his doom).