Tanar'ri is a term encompassing several races of beings that dwell on worlds and realities throughout the multiverse. The name is common enough that many an extraplanar or inter-world scholar has heard of the term, but the range of creatures that would ascribe to the label "Tanar'ri" are wildly varied.
Tanar'ri is a term that describes several races that are very different from each other, but a powerful trend among creatures calling themselves Tanar'ri would be the close influence and ties to the universal principal known as Chaos. Some individuals of Tanar'ri heritage feel their ties to chaos and chance calling them to be daredevils or unpredictable but lovable rogues. Other aggregates of Tanar'ri adopt lifestyles of anarchic marauders, and act as brutal and savage anathema toward anything they view as representative of Order. Another important trend among Tanar'ri seems to be an inborn affinity for manipulating and replicating the supernatural forces most humans would categorize as "Magic." This magical aptitude possessed by many Tanar'ri exists in a strength roughly proportional to that creature's form and station. Tanar'ri of similar appearances often have similar abilities, but this cannot be counted upon.
Creatures described as Tanar'ri have wildly varied phenotypes. Some forms of Tanar'ri are disgusting and inhuman patchworks of bone and muscle that possess few, if any, body parts that could remotely be considered anthropomorphic. It should be noted, however, that a not-insignificant number of the creatures that fall under the category of Tanar'ri would be considered highly alluring by the typical human.
While numerous sages might recognize the name and classification "Tanar'ri," it is crucial to remember that such a name refers to a several very different groups of beings with widly separate ethos.
Some realities (like 1337-A) have populations of Tanar'ri that are so sparse as to be vanishingly rare, and are host to institutions interested in intraspecies contact and trade. Thus, some beings might consider the typical Tanar'ri to be a charismatic capable information broker that would intercede on behalf of a close friend regardless of any consequence.
Other realities are host to Tanar'ri whose exploding populations are already beyond count, continually reproducing, and are additionally birthed from souls. Scholars from these numerous realities would doubtless interpret the term "Tanar'ri" as referring to merciless creatures whose unpredictable armies swell to near-infinite size, and rampage around countless worlds as sentient embodiments of ferocity and ruin.
Because many Tanar'ri are powerful, unpredictable, and even the name is not a good indicator of what kind of being one can expect, most mages are loathe to attempt to summon a Tanar'ri.