From the Paleocene to the last days of the Oligocene,  The Titanotheres change from shy forest dwelling midgets to multi-ton giants is one of the finer examples of evolution of a vertebrate group through a relatively short length of time.

The fossil record of the Titanotheres in North America rivals that of horses, rhinos and camels in its step-wise transitions from one form to another with offshoots along the way. One attribute these groups share is the increase in  body mass over time. The last surviving Titanothere species, Titanotherium platyceras shown in the above painting, measured 13 feet long and 8 feet high. The brain however, especially in the case of  the Titanotheres remained small. 

During their development Titanotheres were browsing animals, having evolved in the lush forested environments of the Eocene world. Processing leafy greens requires a molar with low rounded cusps, not a very specialized condition. This would soon have to change 

With the global environmental upheaval of the Eocene-Oligocene transition, the Titanotheres world became cooler and drier. Grasses became a more dominant plant type as forests retreated towards the river valleys and arid grassy scrublands expanded. Herbivores in this new open environment needed to adapt physiologically in a few basic ways to avoid being killed by predators. One is to become very fast, like the horses did, or become very large and powerful like the Titanotheres. The critical physical adaptation that had to happen for herbivores to survive in the Oligocene is being able to make use of the dominant emerging food plant: grass. 

The wind swept plains adhere a lot of dust and sandy particles to the grass blades making it a very tough chew. Molars had to change by folding the cusps in on themselves, bringing the hardest part of the tooth; the outer dentine, inward to form sharp transverse ridges to become the instrumental element of the grinding platform, which took on the look of a coarse file or washboard.

The Titanotheres had begun to slowly change their tooth structure to this end, there was still shrubs to browse on, but the inexorable change from the Oligocene scrubland to full blown grasslands in the Miocene was forcing one of the issues of adapt or die, and the Titanotheres never saw the Miocene grasslands .

One of the most abundant fossils of the Oligocene plains; the last surviving Titanotheres, had also adapted behaviorally to life on the plains: living in herds. More ears, eyes, and noses to keep a look out for predators had obvious benefits to survival. In the case of horses, once a threat is recognized and the alarm sounded, running was the preferred method of avoidance. Tintanotheres went for size however, so perhaps like Muskoxen they formed a protective circle around juveniles and older members of the herd with the strongest individuals forming the outer ring of defense. This outer ring was bristling with horns of bone encased with a rock hard leathery hide powered by a massive musculature to drive the point home; "come closer and be crushed!"

With all of this evolution taking place for the herbivores to survive life on the plains, predators too had extreme challenges to keep up in the arms race. Living in groups, physical adaptation for speed and strength was also occurring in predators. Hyaenodon horridus  was adapting its limbs for a more speedy distance covering mode of existence, which has obvious benefits for life on the vast expanses of the arid scrubland. 

One advantage predators tend to have over herbivores is a larger brain. Oftentimes predators have to respond with several types of strategies based on recognition of different situations to procure their food. Herbivores do not have to expend as much mental activity to get sustenance. This fact does not apply to every carnivore and herbivore, it is a generalization.

It is not known exactly if Hyaenodon lived in packs, the size of its brain case does not give one confidence that it was of particular cunning, and the Creodonts were soon approaching their swan song in their history on this planet, presumably losing ground to the more efficient and adaptable modern carnivore groups.

But perhaps Hyaenodon had strategies to deal with situations like the Titanothere defense ring, especially when times were tough and for whatever reason easier prey, such as Oreodonts were less plentiful. The painting above shows Hyaenodons poking and prodding at the Titanotheres to panic and confuse them into breaking formation so the calfs can be separated out, a hazardous operation at best, you could even say it is a dumb idea.