Demon

Demons (often interchangeable with Monsters) are evil spirits that are opposed to God, God's people, and/or generally terrorize the populace.

Vampires
Vampires are the main characters and antagonists of the Hellsing series. They are supernaturally strong and fast being that hunt and feed on humans, often controlling a larger army ghouls. Alucard is the most prominent vampire and a true vampire, followed by Seras, the Draculina.
 * ''For main article go here..

Werekin
Werekin are humans that exhibit animal-like qualities. There are very few Werekin ever seen in the Hellsing universe. Only two were ever known: The Captain and Schrödinger, a Werewolf and a Werecat respectively. Possibly the only two, they've thrown their lot in with the Millennium Organization.
 * ''For main article go here.

Ghouls
Ghouls are the red-shirts of monsters. They are undead and totally obedient to their makers, the Vampire who bit them. They are a step above zombies, with the ability to control weapons and work as functional army, however they still hunger for human flesh and still have the same viral properties zombies do. Shooting them in the heads or hearts spells death for them.
 * ''For Main article go here.

Other Species

 * Humans