I'd just say that the frothing jarbo gets rage as per the dire wolverine (for ex).
The problem with Rage is that a Jarbo only has 3 hit points, so is unlikely to survive the attack it needs to provoke its rage.
I'd prefer a Frenzy that triggers whenever it enters combat, maybe adapted from the Sahuagin's Blood Frenzy.
Blood Frenzy: Once per day a sahuagin that takes damage in combat can fly into a frenzy in the following round, clawing and biting madly until either it or its opponent is dead. It gains +2 Constitution and +2 Strength, and takes a -2 penalty to Armor Class. A sahuagin cannot end its frenzy voluntarily.
Perhaps something like this:
Frothing Frenzy (Ex) #1: Once per day, a frothing jarbo that enters combat can fly into a frenzy in the following round, biting madly until either it or its opponent is dead. It gains +2 Constitution and +2 Strength, and takes a -2 penalty to Armor Class. A jarbo cannot end its frothing frenzy voluntarily.
I'd probably modify the bonuses though, maybe borrow some inspiration from the Dragon #318 Liopleurodon.
Frenzy (Ex): If a liopleurodon is reduced to less than half its normal maximum hit points, it flies into a terrible rage of frenzied activity. During a frenzy, the liopleurodon gains a +4 bonus to its Armor Class and attack rolls. Its swim speed gains a +30 foot bonus, and if it makes a full attack action it gains an additional bite attack. The frenzy persists for 1 minute, after which the liopleurodon becomes fatigued. A liopleurodon cannot enter a frenzy while it is fatigued.
Frothing Frenzy (Ex) #2: A frothing jarbo that enters combat will fly into a frenzy in the following round, biting madly until either it or its opponent is dead. It gains a +2 bonus to its Armor Class and Strength, plus a +10 foot bonus to its speed. if it makes a full attack action it gains an additional bite attack. The frenzy persists for 1 minute, after which the jarbo becomes fatigued for 1 hour. A jarbo cannot end its frothing frenzy voluntarily or enter a frothing frenzy while it is fatigued.
I'd like to make the disease transmitted more interesting than filth fever too, if possible. Maybe cackle fever, since it does Wis damage, which seems thematically appropriate for a rage-causing disease.
I like the Wisdom damage idea. A filth fever based version was problematic since the ability damage it does to an infected Jarbo impairs its combat abilities.
Disease (Ex): Any creature bitten by a frothing jarbo is exposed to frothing fever. A jarbo that take Wisdom damage from frothing fever become a frothing jarbos until it dies or recovers from the disease.
Frothing fever—bite, Fortitude DC 11, incubation period 1d3 days, damage 1d3 Wis. The save DC is Constitution-based.
I'm wondering about adding a +2 racial bonus to the DC.
