I did the math. Monolorhino wins if, and only if, it knows to rely on double distraction.
Here’s my first run-through, which would’ve happened if I commanded both with my lack of experience on these beasties.
Monolorhino: Distracting Impact (1350 damage, 3150 Mammoth HP remaining)
Mammoth: Persistent Ferocious Strike (1190 damage, 2800 Rhino HP remaining)
Monolorhino: Definite Impact (1500 damage, 1650 HP remaining)
Mammoth: Armor Piercing Rampage (2800 damage, Rhino at 0 HP)
Here’s the rhino if it knows what it’s doing:
Monolorhino: Distracting Impact (1350 damage, 3150 Mammoth HP remaining)
Mammoth: Persistent Ferocious Strike (1190 damage, 2800 Rhino HP remaining)
Monolorhino: Distraction (900 damage, 2250 HP remaining)
Mammoth: Armor Piercing Rampage (1400 damage, 1400 HP remaining)
Monolorhino: Definite Impact (1500 damage, 750 HP remaining)
Mammoth: Persistent Ferocious Strike (1350 damage, 50 HP remaining)
Monolorhino: Distracting Impact (1350 damage, mammoth down by 600 HP)
But if they’re both really smart and the mammoth predicts double distraction, the rhino still wins:
Monolorhino: Distracting Impact (1350 damage, 3150 Mammoth HP remaining)
Mammoth: Persistent Ferocious Strike (1190 damage, 2800 Rhino HP remaining)
Monolorhino: Distraction (900 damage, 2250 HP remaining)
Mammoth: Persistent Ferocious Strike (1190 damage, 1610 HP remaining)
Monolorhino: Distraction (900 damage, 1350 HP remaining)
Mammoth: Persistent Ferocious Strike (1190 damage, 420 HP remaining)
Monolorhino: Definite Impact (1500 damage, mammoth down by 150 HP)
The fact that we have a close fight between a unique and an epic is, however… Well, you know what it is. Even GamePress ranked it in the Low Alpha tier with Ankylocodon, Rajakylosaurus, and Arambourigiania.