I have one question: what is the Total Character Level / Hit Dice of a monster cohort?
For example, assume I take the Leadership feat at level 8 to gain a Giant Eagle cohort. A Giant Eagle has 4 HD and a cohort level of 6. When he gains a level for the first time (gaining a level of fighter) I want to take the Toughness feat for him.
How many HP does the cohort gain?
A) As a 1st-level-fighter, he gains 3 HP.
B) As a creature with 4 HD plus one level of fighter, he gains 5 HP.
C) As his cohort level is now 7, he gains 7 HP.
I have one question: what is the Total Character Level / Hit Dice of a monster cohort?
For example, assume I take the Leadership feat at level 8 to gain a Giant Eagle cohort. A Giant Eagle has 4 HD and a cohort level of 6. When he gains a level for the first time (gaining a level of fighter) I want to take the Toughness feat for him.
How many HP does the cohort gain?A) As a 1st-level-fighter, he gains 3 HP.
B) As a creature with 4 HD plus one level of fighter, he gains 5 HP.
C) As his cohort level is now 7, he gains 7 HP.
There is no accurate flat formula. To find the true value you have to analyze each monster individually, and another factor is that some abilities that are good at lower level are not so good later on. This means the monster that was once worth a +5 LA may later be only good for a +3, but you may still be paying for the +5.
I just realized I did not answer your question, but since its good advice I will leave it. You only get exta HP based on actual HD, not your effective level. This means the eagle get 5 HD.
Also, keep in mind that the bestiary's incomplete rules on monsters as player characters (which monstrous cohorts are very similar to, especially when you start giving them class levels) suggests doing an LA buyoff, as it were.
Quoted from the PRD:
Note that in a mixed group, the value of racial Hit Dice and abilities diminish as a character gains levels. It is recommended that for every 3 levels gained by the group, the monster character should gain an extra level, received halfway between the 2nd and 3rd levels. Repeat this process a number of times equal to half the monster's CR, rounded down. Using the minotaur example, when the group is at a point between 6th and 7th level, the minotaur gains a level, and then again at 7th, making him a minotaur barbarian 4. This process repeats at 10th level, making him a minotaur barbarian 8 when the group reaches 10th level. From that point onward, he gains levels normally.
I'm not fully certain that this rule applies to monstrous cohorts, but I would allow it for creatures whose CR is largely determined by its hit dice or physical ability score.