The heart near the health bar isn't a rage meter like the giga it's a momentum meter for his charge and when it stop flashing and stays a solid red that means it's fully charged

Tamed rhinos do more charge damage than wild ones.

When shooting it ride a bird so it doesn't give you an anal fissure when it shoves its 7ft long horn in your butt

If you tame a high level and upgrade melee and stamina it is possible to enrage a giga with only a few full speed dashes