D&D helped me be able to identify this dinosaur for my Geology class in college. Thank you D&D.

A simple Ankylosaurus. Usable for the Polocanthus, Sauropelta, and other ankylosaurids.

The only change that this guy got was in its tail attack. It used to have only one Tail attack, but the attack did 18(4d6+4) bludgeoning with a DC 14 Strength save or be knocked prone. Now, it has two attacks that deal 9(1d10+4) each, and it automatically knocks huge or smaller creatures prone. Sadly, the Indominus Rex is Gargantuan, so this ankylosaurus do be dead. 

Fusion time! To really instill the fear of an Ankylosaur in your players, raise the Tail's damage to 18(4d6+4), but still have it attack twice. 

Quote from Arsen_01 >>

A simple Ankylosaurus. Usable for the Polocanthus, Sauropelta, and other ankylosaurids.

The only change that this guy got was in its tail attack. It used to have only one Tail attack, but the attack did 18(4d6+4) bludgeoning with a DC 14 Strength save or be knocked prone. Now, it has two attacks that deal 9(1d10+4) each, and it automatically knocks huge or smaller creatures prone. Sadly, the Indominus Rex is Gargantuan, so this ankylosaurus do be dead. 

Fusion time! To really instill the fear of an Ankylosaur in your players, raise the Tail's damage to 18(4d6+4), but still have it attack twice. 

 Just be ready for the party druid to make your life miserable once she takes that form. That 4d6+4 will shred your encounters, especially if you also keep the knockdown rider.