Dark creatures differ from other animals in several important ways.

• They are magical in nature.
• They often do not have a complete life cycle the way a typical animal would but exist simply as a physical extension of an evil intention.
• They seek actively to harm, often for no other reason than that's what they do (i.e., not for reasons of sustaining their own lives).

Dark creatures are not animals. They are actually another form of magical being. Fantastic Beasts refers to many of them as "demons," not in a religious sense, but because it describes a key aspect of them all: they exist to actively do harm and damage. This harm is different from the harm that, say, a manticore would do, since the manticore would be attacking someone to eat them. In other words, it is a predator and that's what predators do to survive.

Dark creatures, on the other hand, aren't animals. They don't have life cycles in the same sense that normal animals have. They attack for the sake of hurting someone, not simply to eat. The Red Caps are a great example. They bludgeon unususpecting travellers so that they can use their blood to stain their cloth caps red. It doesn't give them food, it doesn't allow them to reproduce, they do it purely out of nastiness. One way of thinking about it is to say that they are a physical embodiment of an evil, harmful intent. And if you think about the essence of magic being intent, Dark creatures are a physical embodiment of Dark Magic.

The lines do get blurry, surely. But even werewolves fit this description. Most of the time they aren't Dark creatures, but when they are, they attack people with the intention of passing the disease on to others or kill them.

As for the pixies, they aren't Dark creatures. That's why Seamus snickered when Lockhart showed up with them. They're simply magical creatures with a rude attitude.

some Dark creatures:

• Banshees
• Boggarts
• Dementors
• grindylows
• hinkypunks
• kappas
• Pogrebin
• Red Caps
• vampires (considered part-human)
• werewolves (although human part of the time)

Allied with Lord Voldemort, dementors are perhaps the darkest creatures of all [more...]

The Demiguise is a peaceful, herbivorous creature that can make itself invisible. It resembles an ape with large, black eyes and long, silky hair. This hair can be woven into Invisibility Cloaks. The Demiguise is native to the Far East (FB).

This plump, flightless bird escapes danger by vanishing in a burst of feathers. Muggles knew this bird as the "dodo" and believe that it is extinct, being unaware of its ability to vanish at will. Because this belief (and associated guilt) has spurred more enlightened attitudes toward the animal world among many Muggles, wizards have encouraged it (FB).

When Hermione buys Crookshanks at the Magical Menagerie, a wizard is inside being advised on double-ended newts.  Aside from this, we don't know what they are (PA4).

The Doxy is a small fairy-like creature that is covered with black hair. They have sharp venomous teeth (FB). Doxies are pests. They can infest houses, taking up residence in the draperies. Removing them requires a good supply of Doxycide. It's a good idea to have an antidote for Doxy venom on hand as well (OP6).

Dragons, large flying reptiles that breathe fire, are some of the most awe-inspiring and dangerous of all magical beasts. There are ten varieties of dragons in the world today (more...)

• Antipodean Opaleye
• Chinese Fireball (Liondragon)
• Common Welsh Green
• Hebridean Black
• Hungarian Horntail
• Norwegian Ridgeback
• Peruvian Vipertooth
• Romanian Longhorn
• Swedish Short-Snout
• Ukrainian Ironbelly

This strange creature looks like just another hunk of dead wood floating in the marshes in which is lives. The Dugbog has finned paws and sharp teeth with which is attacks small mammals. The Dugbog is particularly fond of Mandrakes (FB).