The only known specimen was discovered by high school student Guillermo Aguirre-Zabala in Comallo, in the region of Patagonia, and was made the holotype of the new genus and species Kelenken guillermoi in 2007.

The long and slender tarsometatarsus of Kelenken suggests that it could run faster than had previously been assumed for large phorusrhacids, and would have been able to chase down small animals.

Kelenken is known from the Collón Curá Formation, and lived during the Colloncuran age of South America, when open environments predominated, which allowed more cursorial (adapted for running) and large animals to occur.

[3] Prior to the animal receiving a scientific name, the specimen was reported and discussed by the Argentine paleontologists Luis M. Chiappe and Sara Bertelli in a short 2006 article.

[3][4] In 2007, Bertelli, Chiappe, and Claudia Tambussi made the specimen the holotype of Kelenken guillermoi; the genus name refers to a spirit in the mythology of the Tehuelche people of Patagonia which is represented as a giant bird of prey, and the specific name honors its discoverer.

The skull of the latter disintegrated during collection (leaving only the tip of the beak), which hampered comparison between phorusrhacid taxa of different sizes, until the discovery of Kelenken.

Five phorusrhacid subfamilies were recognized at the time (Brontornithinae, Phorusrhacinae, Patagornithinae, Mesembriornithinae and Psilopterinae), though their validity had not then been confirmed through cladistic analysis, and the describers found Kelenken most similar to taxa that had traditionally been considered phorusrhacines.

Features shared with phorusrhacines include that the hind part of the skull is low and compressed from top to bottom, a wide occipital table, a blunt postorbital process, and a tarsometatarsus that is similar to that of Titanis in that the supratrochlear surface of the lower end is flat.

[3][5] The Brazilian paleontologist Herculano Alvarenga and colleagues published a phylogenetic analysis of Phorusrhacidae in 2011 that found Kelenken and Devincenzia to be sister taxa, each other's closest relatives.

[6] In their 2015 description of Llallawavis, the Argentinian paleontologist Federico J. Degrange and colleagues performed a phylogenetic analysis of Phorusrhacidae, wherein they found Phorusrhacinae to be polyphyletic (an unnatural grouping).

They also appeared in North America at the end of the Pliocene, during the Great American Biotic Interchange, and while fossils from Europe have been assigned to the group, their classification is disputed.

It is unclear where the group originated; both cariamids and phorusrhacids may have arisen in South America, or arrived from elsewhere when southern continents were closer together or when sea levels were lower.

These reconstructions highlighted their assumed very tall beaks, round, high eye sockets, and vaulted braincases, but Kelenken demonstrated the significant difference between the skulls of large and small members of the group.

This fusion makes it difficult to identify how these bones were part of the skull roof, but the blunt, robust postorbital processes were probably mainly formed by the frontals.

The postorbital process contains scars left by massive jaw muscles, parts of which invaded most of the skull roof at the level of the parietal bones.

The foramen magnum (the large opening at the base of the skull through which the spinal cord enters) is almost triangular, uniquely for this genus, and has a blunt upper apex, and it is slightly smaller than the condyle.

The distal vascular foramen, an opening on the lower front side of the tarsometatarsus, has a centralized position, above the upper ends of the third and fourth trochleae.

These researchers pointed out that the narrowing of the pelvis, upper maxilla and thorax could have been adaptations to enable the birds to search for and take smaller animals in tall plant growth or broken terrain.

[5] In 2005, Rudemar Ernesto Blanco and Washington W. Jones examined the strength of the tibiotarsus (shin bone) of phorusrhacids to determine their speed, but conceded that such estimates can be unreliable even for extant animals.

This strength could be used for accessing the marrow inside the bones, or by using the legs as kicking weapons (like some modern ground birds do), consistent with the large, curved, and sideways compressed claws known in some phorusrhacids.

[13] According to Chiappe and Bertelli in 2006, the discovery of Kelenken shed doubt on the traditional idea that the size and agility of phorusrhacids correlated, with the larger members of the group being more bulky and less adapted for running.

[4] In a 2006 news article about the discovery, Chiappe stated that while Kelenken may not have been as swift as an ostrich, it could clearly run faster than had previously been assumed for large phorusrhacids, based on the long, slender leg-bones, superficially similar to those of the modern, flightless rhea.

[1] In another 2006 news article, Chiappe stated that Kelenken would have been as quick as a greyhound, and that while there were other large predators in South America at the time, they were limited in numbers and not as fast and agile as the phorusrhacids, and the many grazing mammals would have provided ample prey.

Chiappe stated that phorusrhacids crudely resembled earlier predatory dinosaurs like Tyrannosaurus, in having gigantic heads, very small forelimbs, and very long legs, and thereby had the same kind of meat-eater adaptations.

[15] A 2010 study by Degrange and colleagues of the medium-sized phorusrhacid Andalgalornis, based on Finite Element Analysis using CT scans, estimated its bite force and stress distribution in its skull.

Due to the relative weakness of the skull at the sides and midline, these researchers considered it unlikely that Andalgalornis engaged in potentially risky behavior that involved using its beak to subdue large, struggling prey.

[9] Kelenken was discovered in pyroclastic (rocks ejected by volcanic eruptions) outcrops belonging to the Collón Curá Formation in the southeastern corner of Comallo, Patagonia, an area covered in whitish tuffs.

[18] The Collón Curá Formation and the Colloncuran age of South America represent a time when more open environments with reduced plant covering predominated, similar to semi-arid and temperate to warm, dry woodlands or bushlands.

[20] The Collón Curá Formation of Argentina has provided a wide assemblage of mammals, including at least 24 taxa such as the xenarthrans Megathericulus, Prepotherium, Prozaedyus and Paraeucinepeltus, the notoungulate Protypotherium, the astrapothere Astrapotherium, the sparassodonts Patagosmilus and Cladosictis, the marsupial Abderites, the primate Proteropithecia, and rodents such as Maruchito, Protacaremys, Neoreomys and Prolagostomus.