Acrocanthosaurus [PvE Cross]

Acrocanthosaurus is a tank-focused theropod, tougher than a Rex and built around its Shield Stance rather than brute force.
Toggle Shield Stance and it turns to take hits on its armored side, halving incoming damage while it moves sideways to keep that side facing the threat. Damage taken while in this stance fills an Adrenaline bar.
Once Adrenaline is full, popping Rage Mode roars back enemies and boosts attack power, movement speed, and health regen for a short window - so it heals itself as part of going aggressive, not as a separate defensive option.
 
Due to its powerful attacks, defensive stance and healing ability, it's considered to be the new best boss mount.