Gigantaurok
With no competition for larger herbivore niches, a population of Centauroks split off and dipped their toes into gigantism. Quadrupling in size, the Gigantaurok is so large that it has developed a high internal body temperature and therefore high metabolism through gigantothermy. Despite its huge size, it is still not quite at the theoretical largest size for arthrotheres, though it is still pretty massive, being comparable in size to a Terran elephant. To support its giant size without running out of room for muscles, the Gigantaurok supports its weight on the endoskeleton which exists under its exoskeleton, with the exoskeleton itself only being used for muscle attachment so that it does not have to become too thick to carry any muscles. Its giant size allows it access to food its ancestor could not reach and makes it difficult for any predator to take down at the time it evolved. Its neck-torso segment is slender and has gained additional articulation, as it would otherwise be unable to bend down to drink. It has reduced or lost a lot of spikes to reduce weight; this has the effect of making it even more centaur-like than before in silhouette, though the shape of its head continues to ruin the illusion.
The Gigantaurok uses its enormous size to feed on equally enormous flora, filling a high-browser niche much like a Terran giraffe or sauropod. Its long arms also grant it access to flora even higher above its head, increasing its feeding envelope without forcing it to have an insanely elongated upper body. Its eating method mainly consists of getting as much food into its big fermenting gut as possible. Its large fins, in addition to sexual display and communication, are now also used to release excess heat—not unlike the ears of an elephant—in the heat of summer in the warmer parts of its range. Because of how big it is, it is no longer cursorial, and some of its limb segments—specifically on its feet—have fused. Notably, because of its evolutionary history, it has horse-like feet on its hind legs and deer-like feet on its front legs—causing its trackways to look very mismatched.
With its huge size, the natural downwards tilt of the Gigantaurok’s abdomen is not enough to ensure little injury to its young when giving birth, so it has developed the ability to protrude part of its birth canal out of its gonopodium to form a sort of “birth slide”. Like its ancestor, it is viviparous, and its gonopodium is not fully fused in females to allow expansion for birth. It does not engage in parental care, but it is social towards others of its kind and can often be found in loose groups of individuals of similar size or age.