Taken from the Creature Codex
This snake is whip thin and longer than a man is tall. Its tail bears a strange callused pad.
Although they are commonly mistaken for ordinary venomous snakes, hoop snakes are especially feared in regions where they live. If spotted in time, an ordinary snake is easy to outrun and usually strikes only to defend itself. Not only are hoop snakes fast, they sometimes intentionally hunt humanoids. Although a hoop snake is too small to eat a human, they feed primarily on rats, carrion birds and other creatures attracted to decaying flesh. If no corpses are available in its habitat, it will gladly make its own to serve as bait.
By grabbing its tail in its mouth and rolling like a wagon wheel, hoop snakes can sprint as fast a horse can run. Most hoop snakes only use this ability when hunting prey or fleeing a predator, however—the hoop form is decidedly noticeable and draws the attention of people and predators alike. In battle, a hoop snake latches onto its victim with poisonous fangs and wraps around the creature, injecting venom and squeezing it to death simultaneously.
Hoop snakes grow to ten feet long. Different hoop snakes have different colorations, but they tend to the bright and garish rather than dull and camouflaged. Blue with brown lateral stripes is the most common coloration, followed by red, yellow and black banded. Like most snakes, hoop snakes are solitary for most of their lives, but will cluster in huge numbers to hibernate during the winter.
Hoop snakes are more intelligent than the average snake, possessing canniness closer to that of dogs or cats than most reptiles. As such, some folk seek to domesticate them and use them as guard animals—a tricky proposition. Hoop snakes are fond of the taste of milk, occasionally latching onto the udders of livestock and drinking them dry, so fresh milk and lots of it is often the key to training a hoop snake.
Hoop Snake Animal Companions
Starting Statistics: Size Small; Speed 20 feet, climb 20 ft., swim 30 ft.; AC +1 natural armor; Attack bite (1d3),; Ability Scores Str 11, Dex 18, Con 10, Int 2, Wis 15, Cha 8; Special Qualities low-light vision, hoop form (40 ft.), poison (1d3 Con), scent
4th-Level Advancement: Size Medium; AC +1 natural armor;Ability Scores Str +4, Con +2; Special Qualities constrict (1d6), grab, hoop form (60 ft.), poison (1d4 Con)
Hoop Snake CR 3
N Medium animal
Init +3; Senses low-light vision, Perception +7, scent
AC 16, touch 13, flat-footed 12(+2 natural, +3 Dex, +1 dodge)
Fort +5, Ref +7, Will +3Offense
Speed 20 ft., climb 20 ft., swim 20 ft.; 50 ft. while in hoop form
Melee bite +5 (1d4+3 plus poison plus grab)
Special Abilities constrict (1d6+3)
Str 15, Dex 16, Con 13, Int 2, Wis15, Cha 8
Base Atk +3; CMB +5 (+9 grab); CMD 19
Feats Dodge, Mobility
Skills Climb +14, Perception +7, Stealth +4, Swim +14
SQ hoop form
Environment temperate plains and hills
Organization solitary or knot (2-40)
Hoop Form (Ex) As a move action, a hoop snake can curl itself into a ring by grabbing its tail in its mouth. When it is in hoop form, its move speed becomes 50 ft., but it cannot use its climb or swim speeds. A hoop snake making a charge in hoop form gains a +4 bonus to its attack roll. A hoop snake in hoop form suffers a -8 penalty to Stealth checks. As soon as a hoop snake makes an attack, it drops out of hoop form as a free action. A hoop snake can leave hoop form at any time as a free action.
Poison (Ex) bite—injury; save Fort DC 13, frequency 1/round for 4 rounds, effect 1d4 Con, cure 2 saves. The save DC is Constitution based.