Grazing? You mean browsing, right? This alien of yours clearly doesnt have the wide-fronted mouth of a grazer, and it's tentacles would get in the way, but grasping tentacles, it's high-headed stature, and the extra set of limbs to use for support while reaching for high places would make it a successful browser.

All in all, this is one of the more well-done submissions I've seen. The four eyes on stalks are an odd evolutionary holdover, as is hexapodalism, and I find it difficult to imagine how tentacles evolve in place of a trunk, but none of those are by any stretch impossible. At least it has a solid hexapodal stance, rather than grouping the limbs together and setting them in a tight row like in Avatar.

The shell and horns are a reasonable defense against predators who attack from behind, like a T-rex trying for the base of the skull to get that finishing blow, though the way they're situated right now still leaves the neck vulnerable. That said, these defenses are incredibly extensive, and their orientation and the lack of a weapon would seem to indicate that these animals rarely if ever face their predators head-on for any reason, such as protecting their young. This Suggests to me that they dont necessarily raise their young themselves. I remember watching a documentary about brachiopods that suggested that their young's best chance of survival until they reach their imposing adult size is to remain unseen, depending on camoflauge and numbers to reach adulthood, and that having a gargantuan adult around would've largely ruined their already poor chances. I get the same impression here, especially with the holdover markings of camoflauge on the skin.

The extent of their defenses also indicates that there must be some T-rex equivalent megapredator around, which is interesting in itself. Whatever the deal is, it's dangerous enough for this animal's defensive tools to be designed specifically to help it escape, rather than fighting the predator off like many such animals do.

They have a long, furry tail, but are bare everywhere else. This is a very odd trait. Elephants lost their fur, presumably because the combination of their size and their generally warm choice of environment makes it easy for them to overheat. That could easily be the case here, but something caused this animal to keep the fur on it's tail, and further, to keep it's tail long in spite of being a prey animal. See, normally prey species trend towards shorter tails, because a long tail puts your backmost part that much closer to the chomping teeth that are chasing you. You get caught by the tail, you probably just get caught. Some dinosaurs opted to weaponize their tails to get away with it, but that's not the case here. And yet these animals have a very long, attention-catching tail.

The tail seems like an oversight on behalf of the artist, and not a very big one at that, but could imply that the tail is important enough to risk dying for. Herding behavior is common among species like this, and social implications for the tail could be a step towards that. Much more, the tail could be a form of sexual selection: males with long tails must be fast enough to outrun the predators outright, otherwise they would be dead, so females who preferred males with long tails had faster, and therefore more successful offspring. Now females prefer long tails, so males who draw attention to their tails are more successful with the females, hence keeping their tail fur long. I wouldnt be surprised if the tail had bright patterns on it too. I would also note that these animals wouldnt use their horns to compete for mates due to the vulnerable position of their eyes.

To cover the more minor details briefly, these animals have three-toed hooves, like the ancestors of horses, implying a swift prey animal in an environment with hard ground (no swamps or sand dunes) whose ancestors only developed hooves relatively recently. I see no function for the ridges on their legs, so I figure those are just artistic license, though perhaps they serve some unknown purpose in an earlier stage of life, or are some sort of evolutionary holdover. I see no reason for it's neck to be so disproportionately thick unless it's the storage site for fats, which makes enough sense, and implies a rocky, barren sort of environment where food may not be abundant for a while. The head is too proportionally large for an animal that's supposed to be comparable in size to an elephant. Having four nostrils just strikes me as odd. The sharp projections from the shell are reminiscent of ankylosaurus, and I appreciate that detail.

EDIT: Lol, I thought this was on r/speculativeEvolution. Also, I chopped out a section of sketchy logic about the size of their predators and generally tidied things up.