Snakes have the following traits.
Variable Size: See Table: Creature Sizes for attack and Defense modifiers based on size, as well as for a snake’s fighting space (assuming the creature is coiled) and reach.
Improved Grab (Ex): Only constrictor snakes have this ability. To use this ability, a constrictor snake must hit with its bite attack. If it gets a hold, it can constrict (see below). See Improved Grab.
Poison (Ex): A viper injects venom with a successful bite. The victim must succeed on a Fortitude save or take 1d6 points of temporary Constitution damage; a second Fortitude save must be made 1 minute later to negate the venom’s secondary damage (same as the initial damage). The save DC is equal to 11 + 1/2 the snake’s Hit Dice + the snake’s Constitution modifier (DC 11 for a Tiny viper).
Skill Bonuses: Snakes receive a +8 species bonus on Balance checks and a +4 species bonus on Hide, Listen, and Spot checks. Snakes apply either their Strength or Dexterity modifier, whichever is higher, to Climb checks.
Bonus Feat: Vipers gain the bonus feat Weapon Finesse (bite).
Constrictor Snake: CR 2; Medium-size animal; HD 3d8+3; hp 16; Mas 13; Init +3; Spd 20 ft., climb 20 ft., swim 20 ft.; Defense 15, touch 13, flat-footed 12 (+3 Dex, +2 natural); BAB +2; Grap +5; Atk +5 melee (1d6+4, bite); Full Atk +5 melee (1d6+4, bite); FS 5 ft. by 5 ft. (coiled); Reach 5 ft.; SQ improved grab, constrict 1d6+4, scent, low-light vision; AL none; SV Fort +4, Ref +6, Will +2; AP 0; Rep +0; Str 17, Dex 17, Con 13, Int 1, Wis 12, Cha 2.
Advancement: 4–8 HD (Large); 9–16 HD (Huge); 17–32 HD (Gargantuan).
Tiny Viper: CR 1/3; Tiny animal; HD 1/4 d8; hp 1; Mas 11; Init +3; Spd 15 ft., climb 15 ft., swim 15 ft.; Defense 17, touch 15, flat-footed 14 (+2 size, +3 Dex, +2 natural); BAB +0; Grap –10; Atk +5 melee (1d3–2 plus poison, bite); Full Atk +5 melee (1d3–2 plus poison, bite); FS 2 1/2 ft. by 2 1/2 ft. (coiled); Reach 0 ft.; SQ poison, scent, low-light vision; AL none; SV Fort +2, Ref +5, Will +1; AP 0; Rep +0; Str 6, Dex 17, Con 11, Int 1, Wis 12, Cha 2.
Feat: Weapon Finesse (bite).
Advancement: 1/2–1 HD (Small); 2 HD (Medium-size); 3–4 HD (Large); 5–16 HD (Huge).