Dark-type Pokémon were added in Generation II.
Dark-type Pokémon are notable for the fact that they don't have any strong stat trends other than Attack (which is in and of itself modified by a few powerful outliers and slightly ironic as Dark was a special type, not a physical type.), but usually have relatively poor defenses, with the exception of Umbreon and Tyranitar (the second is probably due to Template:T2). Just the same, Pokémon that are both Dark-type and Template:Type2 have no true weaknesses, as the resistances of the Dark-type cover the weaknesses of Ghost, and vice-versa. One of the greatest advantages of the Dark-type is that they are immune to Template:Type2s.
Dark-type moves, when combating Psychic-types, are especially useful due to their secondary effects; Crunch lowers Defense (an already low stat among Psychic-types), Bite causes Flinching, Faint Attack never misses, while Pursuit hits Pokémon as they switch for double damage. Many Dark-type moves also involve stealing or deception, such as moves like Thief, which steals an opponent's held item, and Snatch, which steals the beneficial effects of an opponent's moves.
Interestingly, despite the fact that in generations prior to Generation IV, all Dark-type moves were special moves, there is only one special Dark-type move in Generation IV (Dark Pulse), which itself was actually introduced in that generation, meaning all Dark-type moves from before are actually considered physical moves.
When used in contests, Dark-type moves typically become Smart-type, but can also be any of the other four Contest types.
In total, there are 23 Pokémon with the Dark-type.
Most Dark Pokémon in the anime appear to fall into one of two groups in terms of personality. The first group are michievous, sneaky Pokémon, who tend to be vengeful. For example, the Sneasel in Hail to the Chef, who got revenge on TRMeowth ([[{{{2}}} (move)|{{{2}}}]]) for some rude comments by sabotaging Meowth's food, or the Nuzleaf in Turning Over a Nuzleaf who played tricks on Ash and his friends.
The second group are responsible, often solitary Pokémon, who feel obligated to protect someone or something. This is sometimes their own pre-evolution, as seen in Duels of the Jungle when a Weavile was trying to protect a group of Sneasel; or it can be people or a particular place, as seen with Darkrai (anime) in The Rise of Darkrai and Absol in Absol-ute Disaster. (Note: Protecting a place or certain people/Pokémon is also typical of Legendary Pokémon, of which Darkrai is one.)