"Monster Hunter" 3.5 monsters

[Debihuman]

I think a unified Monster Hunter boost when the creature loses half its hit points makes the most sense since it would be easy to implement.

This is what the Monster Hunter Wiki had to say:

Rage is when a monster has taken enough Physical Damage and reacts by becoming much more aggressive. Many monsters signal that they are entering an enraged state by roaring but some, like Yian Kut Ku and Gypceros, jump up and down. Those monsters that don't display still have signs that they are enraged, such as markings and/or huffs of smoke coming out their mouths. The attack, defense and agility for each monster significantly increases (to the point that some will one hit KO enemies), as well as certain tolerances (in the case of Diablos and Monoblos, they become unaffected by Sonic Bombs while enraged). It's even possible to use Pitfall Traps on an enraged Rajang or Nargacuga, who normally avoid Pitfall Traps.

How much the stats are increased depends on the monster. Monsters usually get enraged more often when they are close to death, a famous example being the Diablos. Rage can also be induced by waking certain sleeping monsters, and in the case of Yian Kut-Ku, by stunning it with a Sonic Bomb or explosive, or occasionally by breaking a body part (a Shogun Ceanataur becomes 'locked' into Rage when one of its claws is broken).

Most of the time, if a monster is in the middle of an attack and a hunter drives them into rage, they will stop that attack and do the Rage state indication.

Some monsters, such as a Khezu or a Tigrex, sometimes go into a state of "constant rage", in which once that monster is out of rage state, one hit could set it off again.

How's this (I added the last bit to allow for more customization):

Monstrous Rage (Ex): Whenever the creature loses half or more of its hit point, it flies into monstrous rage gaining a Morale bonus to its AC, to attack and damage, and to its saving throws. Creatures with 8 Hit Dice or fewer gain a +2 morale bonus; Creatures with 9-16 Hit Dice gain +3 morale bonus; and creatures with 17 or more Hit Dice gain a +4 morale bonus. In addition, some creatures will have specific monstrous rage abilities or special conditions under which they go into a monstrous rage as noted in their description.