Here is my second creature for review. Please make any comments on changes you think need to be made.

Whipstrike
Medium Magical Beast
Hit Dice: 4d10+4 (26 average hp)
Initiative: +2 (+2 Dex)
Speed: 20 ft.
AC: 16 (+2 Dex, +4 natural)
Attacks: Tail Blade +6, Bite +1
Damage: Bite 1d6+1, Tail Blade 1d6+1
Face/Reach: 5 ft. by 5 ft./5 ft.
Special Attacks: Tail Blade
Special Qualities: Darkvision 60 feet.
Saves: Fort +4, Ref +5, Will +0
Abilities: Str 12, Dex 14, Con 12, Int 4, Wis 10, Cha 6
Skills: Hide +4, Spot +4, Listen +2, Move Silently +6
Feats: Improved Critical (Tail Blade)

Climate/Terrain: Any Forest
Organization: Solitary or Pack (2-16)
Challenge Rating: 4
Treasure: Standard
Alignment: Usually Chaotic Evil
Advancement: -

These creatures look like three foot long rats with large carnivorous jaws and a six foot tail. Near half way down the tail, two razor sharp bone formations protrude from each side and run down the length to the end.

COMBAT
Whipstrikes attack anything they see as they always seem to be hungry. They close on their target using their tail as the primary weapon, trying to kill with one strike.

When attacking in a pack, they like to surround their enemies before they start attacking to try to prevent anything from escaping.

Tail Blade (Ex): The bony projections on the tail act as a razor sharp blade. On a critical hit with the tail, it acts as a vorpal weapon, severing the head of its enemy. This ability does not work on anything size large or bigger. Upon death, the tail loses its vorpal ability.