The ending of Netflix's never uses the word "vampire," it's obvious to audiences that's what the Angel really is.
Believing he has found a god-given blessing that will cure ills, Pruitt adds the Angel's blood into the communion wine, slowly transforming his parishioners who become younger and healthier. In a final horrific homage to the Jonestown massacre, Pruitt and Bev Keane (Samantha Sloyan) eventually serve the entire congregation rat poison during the Easter vigil mass in order to kill them so that they may be resurrected as full vampires.
Pruitt remarks in Midnight Mass that the Angel transforms people into the "best version" of themselves. Ultimately that part seems true: Leeza walks again, Mildred is cured of dementia, and the residents' youth is restored. Regardless of whether the Angel is a vampire or some other monster, one of the questions the series left viewers puzzling over was why it looks so different than the people it turned.
One of the reasons that the vampire differs in appearance from its converts could simply just be a matter of time. Pruitt encounters the vampire in some ancient ruins during a sandstorm, which hints that the creature may have been trapped underground for many, many years. The residents of Crockett Island were freshly turned — so it's possible that they too would have eventually physically transformed in a similar manner, had enough time ed. It is also possible that the vampire could actually be a fallen angel, therefore never human from the start. Pruitt repeats in Midnight Mass that biblical Angels are described as terrifying creatures.
While the origins of the creature are never explained, the inspiration behind its design in Midnight Mass is obvious. Flanagan channels the iconic classic horror film Dracula, shows a similar-looking vampire who is so monstrous because of the centuries spent performing acts of evil. The same could be suggested of Midnight Mass' vampires and why newer converts still look like themselves. Flanagan's latest series obviously isn't the first to capitalize off of the vampire trope, but the religious integration of Catholicism into the story definitely makes Midnight Mass the first of its kind.