For this iteration of the classic monster, I developed powers for nine heads. Additional heads beyond nine look like the first set of heads, in order, and usually bolster the powers of that head.
The mystic hydra heads appear roughly humanoid, but they have broad, toothy mouths that are very inhuman and quite unsettling (I'm picturing the famous old illustration of the manticore - just Google it). The first nine heads are as follows:
Head 1: The head of a red-faced man with a giant mustache. Grants the monster +1 to initiative; heads 10, 19, etc. look the same and add one more point to initiative.
Head 2: The head of a chubby, freckle-faced female druid. Grants the monster protection from good and protection from evil; heads 11, 20, etc. look the same and increase the range of the protection by 10-ft. each
Head 3: The head of a dashing, mustachioed man. Grants the monster charm person three times per day; heads 12, 21, etc. look the same and increase the potency of the spell to charm monster, then suggestion, then command.
Head 4: The head of a turbaned sage with a pronounced overbite. Blood spilled from the monster with four heads turns into a single cobra. Blood is spilled when the monster is struck by a cutting weapon that does at least half its potential max. damage against the monster (i.e. 3-4 points for a dagger, 4-6 points for a short sword, etc.). Heads 13, 22, etc. look the same, and increase the number of cobras created from spilled blood.
Head 5: The head of a hawk-nosed woman with leering eyes. Grants the monster the power of haste; heads 14, 23, etc. look the same and increase the monster's base land speed by 5 feet.
Head 6: The head of a matronly woman with long braids. Grants the monster the ability to cast crushing despair one time per day; heads 15, 24, etc. increase the uses of crushing despair by one per day (or to put it another way, each such head can cast the spell once).
Head 7: The head of an anti-bishop wearing a black mitre. Grants the monster the blink ability, per the blink dog. Heads 16, 25, etc. gain the ability to cast one 1st level cleric spell each.
Head 8: The head of a man with a blue, barbed beard. Grants the monster the ability to spit a 2 dice lightning bolt from this head. Heads 17, 26, etc. look the same and can also spit lightning bolts.
Head 9: The head of a beautiful and terrible elven queen. Victims of this head's bite might be polymorphed (Fortitude save; use the reincarnation table to see what they turn into; lasts for 1d4 days). Heads 18, 27, etc. look the same and have the same power.