Tanar'ri
These are demons, the chaotic evil supernatural badguys, in Advanced Dungeons & Dragons 2nd edition.
Y'see, after TSR got their new CEO Lorraine Williams and Gary Gygax left in disgust, the company decided that nutcases like Jack Chick and Patricia Pulling had to be pandered to, so Williams said "no more demons, or any of that junk. Oh, and get rid of orcs as a playable race, because they're ugly. Ewww." It's marketing genius like this that drove TSR into the ground so that some dinky company whose only product was a card game could step in and buy them out.
In 3e, they wisely decided to use "demon" again, but kept the term "tanar'ri" as a subtype of demon. Since TSR bothered to register "tanar'ri" as a trademark, you will not find this demon subtype in Open Gaming License materials like the d20SRD.
"Fiendish Codex II - Tyrants Of The Nine Hells" states that Devils call all demons Tanar'ri for short, which is strange considering that in the same book it says that in the language of the devils "only one correct way exists to construct any given statement in Infernal." It also states that devils "except when in disguise, they find the urge to correct errors in spoken or written Infernal nearly impossible to resist." This may be a hint as to the origin of Lorraine Williams, since she also insisted on using the term "tanar'ri" instead of "demon."