HYDRA

6-headed—AC 5, HD 6, hp 48, THAC0 13, Hits as +1, Melee bite (1d6 each), Saves 11/12/13/13/14, XP 838 (+700)

7-headed—AC 5, HD 7, hp 56, THAC0 13, Hits as +2, Melee bite (1d8 each), Saves 10/11/12/12/13, XP 1,273 (+825)

8-headed—AC 5, HD 8, hp 64, THAC0 12, Hits as +2, Melee bite (1d8 each), Saves 10/11/12/12/13, XP 1,915 (+1,200)

9-headed—AC 5, HD 9, hp 72, THAC0 12, Hits as +3, Melee bite (1d8 each), Saves 8/9/10/9/11, XP 2,864 (+1,600)

10-headed—AC 5, HD 10, hp 80, THAC0 10, Hits as +3, Melee bite (1d8 each), Saves 8/9/10/9/11, XP 4,120 (+1,800)

11-headed—AC 5, HD 11, hp 88, THAC0 10, Hits as +4, Melee bite (1d10 each), Saves 7/8/9/8/10, XP 5,808 (+2,250)

12-headed—AC 5, HD 12, hp 96, THAC0 9, Hits as +4, Melee bite (1d10 each), Saves 7/8/9/8/10, XP 5,936 (+3,400)

13-headed—AC 5, HD 13, hp 104, THAC0 9, Hits as +4, Melee bite (1d10 each), Saves 5/6/7/5/8, XP 7,972

14-headed—AC 5, HD 14, hp 112, THAC0 8, Hits as +4, Melee bite (1d12 each), Saves 5/6/7/5/8, XP 8,116

15-headed—AC 5, HD 15, hp 120, THAC0 8, Hits as +4, Melee bite (1d12 each), Saves 4/5/6/4/7, XP 10,500

16-headed—AC 5, HD 16, hp 128, THAC0 7, Hits as +4, Melee bite (2d6 each), Saves 4/5/6/4/7, XP 10,660

17-headed—AC 5, HD 17, hp 134, THAC0 7, Hits as +4, Melee bite (2d6 each), Saves 3/4/5/4/6, XP 13,450

18-headed—AC 5, HD 18, hp 142, THAC0 7, Hits as +4, Melee bite (2d8 each), Saves 3/4/5/4/6, XP 13,650

19-headed—AC 5, HD 19, hp 150, THAC0 7, Hits as +4, Melee bite (2d8 each), Saves 3/4/5/4/6, XP 17,700

20-headed—AC 5, HD 20, hp 158, THAC0 7, Hits as +4, Melee bite (2d10 each), Saves 3/4/5/4/6, XP 17,940

Hydrae are gray brown to blackish brown with lighter underbellies tinged towards yellow or tan. Their eyes are amber to orange. The teeth are yellowish white.

Hydrae are reptilian monsters found in marshes, swamps, and similar places, as well as in subterranean lairs. 

Their large, four-legged bodies are surmounted by from 5 to 12 heads (roll an 8-sided die to determine number). Each head has 1 hit die of a full 8 hit points. When all of a hydra's heads are killed, the body dies, but not until each and every head is killed.

The hydra attacks according to the number of heads it has, each head being considered as a hit die. Thus, a hydra of 7 heads attacks as a monster of 7 hit dice. 

It is possible for the hydra to attack several opponents at once, and up to 4 heads are able to attack the same target simultaneously.

Damage scored is based on the number of heads the hydra has: hydrae of 5 or 6 heads do 1-6 hit points of damage/attack, those with 7 to 10 heads score 1-8 points of damage, and hydrae with 11 or 12 heads do 1-10 hit points of damage.

Lernaean Hydra. These creatures are very rare. The lernaean hydra is indistinguishable from a normal hydra until it is attacked. 

Every time one of the heads of these creatures is cut off or killed, it grows two new ones in 1-4 melee rounds unless fire is applied to the dead member or neck stump.

Thus, a 5-headed lernaean hydra could grow to a 12-headed monster in a single combat, gaining the appropriate hit dice and attack potential in the process.

Pyrohydra. Pyrohydrae also appear quite similar to normal hydrae, although their coloration may be tinged with more red. They seldom exceed 7 heads, 10% having 8, and no 9-headed pyrohydra has ever been recorded. 

Each head of a pyrohydra is able to breathe fire twice per day, the breath shooting out in a cone ½" diameter at the mouth, 1" diameter at the end, and 2" long. This fire causes 8 hit points of damage (4 if saving throw versus dragon breath is made). 

They can, of course, strike normally as well.

The dreaded thessalhydrae are monsters which love dark and damp conditions, so they are typically encountered in swamps, jungles, and subterranean lairs.

Normal attack is from the fringe of snake-like heads which ring the vast main mouth of the thessalhydra. Each head in the fringe is some 6 feet long, so many creatures can be attacked during a melee round. Each bite inflicts 1-6 points of damage plus an additional 1-6 unless the victim saves vs. Poison. Fringe heads each take 12 points of damage to sever. Thessalhydrae regenerate lost fringe heads in 12 days. Damage to fringe heads does not harm the body. 

In addition, the main mouth bites for 1-20 points plus another 1-20 points of acid damage unless a saving throw vs. Poison succeeds. Note that items placed in the central maw are usually destroyed, as each owner must save vs. Acid and Crushing Blows every round until the items are removed.

The tail pincer is used to grasp an opponent, inflict 1-12 points of damage in the process, and then deposit the victim in the gaping central maw during the same round.

Once per day a thessalhydra can spit a glob of acidic saliva from its main mouth. The area covered is effectively a 12-foot diameter circle up to 3" distant from the monster. Creatures within this area will sustain 12-72 [12d6] points of acid damage and half damage if a saving throw vs. Poison is successful.

Acids of any sort, including poisonous acids, have no effect upon a thessalhydra.