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

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

Tamed rhinos do more charge damage than wild ones.

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