So, spoilers for all five of you who haven't seen the movie and still care about what happens. The Indominus Rex is a hybrid dinosaur made from combining DNA from the T Rex with velociraptor, tree frog, cuttlefish, and, I don't know, the Rock and I think maybe the Predator. Anyway, it's a bad mofo; smarter and more maladjusted than other dinosaurs, with the ability to camouflage itself. It also temporarily takes over the supposedly trained velociraptor pack because it's essentially a giant velociraptor with an incredibly bad attitude. I suspect How It Should Have Ended got it right when it portrayed the conversation as: I Rex: "Hey, we should eat all the humans." Velociraptors: "You make a very compelling argument."
Here's the thought I had for how this monster could fit into a D&D game, assuming you don't have mad scientists combining a bunch of random DNA together in a lab for giggles. Demogorgon's layer of the Abyss is supposed to have a giant jungle just loaded with dinosaurs and other nasty things. The Indominus Rex could be something that just evolved there, since it is the Abyss. Or, Demogorgon could have made the critter for yucks, because he apparently likes doing that sort of thing. I would imagine that a jungle in the Abyss is a really tough place to live, so while the I Rex probably wouldn't really group together in a less deadly environment, I could see some coming together for mutual benefit in that toxic cesspool. They certainly are smart enough to figure that one out. Obviously, it working with the velociraptors in the movie is what originally suggested the pack tactics. If your party can't handle three or four of these things, though, what are they doing in the Abyss?
Just to be upfront about it, I blatantly stole most of the design from Shawn Ellsworth over at Tribality. They do a lot of good stuff over there, if you haven't checked them out yet. I borrowed a tiny bit from DM Paul Weber, then mixed things up to my liking and here is what I got.
Huge monstrosity, unaligned
Armor Class 15 (natural armor)
Hit Points 147 (14d12+56)
Speed 60 ft.
Str Dex Con Int Wis Cha
25 (+7) 14 (+2) 18 (+4) 7 (-2) 14 (+2) 10 (+0)
Skills Perception +6, Stealth +6
Senses darkvision 60 ft., passive Perception 16
Ambusher. The indominus rex has advantage on attack rolls against any creature it has surprised.
Chameleon. The indominus rex is able to change its color to blend in with the foliage around it. It has advantage on Dexterity (Stealth) checks to hide, and it may take the Hide action as a bonus action.
Keen Senses. The indominus rex has advantage on Wisdom (Perception) checks that rely on sight or smell.
Pack Tactics. The indominus rex has advantage on attack rolls against a creature if it has at least one of the indominus rex’s allies is within 5 feet of the creature and the ally isn’t incapacitated.
Multiattack. The indominus rex makes two attacks: one with its bite and one with either its claws or tail. It can’t make a bite attack and a tail attack against the same target.
Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 33 (4d12+7) piercing damage. If the target is a Medium or smaller creature, it is grappled (escape DC 19). Until the grapple ends, the target is restrained, and the indominus rex can’t bite another target.
Claws. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 20 (3d8+7) slashing damage.
Tail. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 20 (3d8+7) bludgeoning damage.