Do not be distracted with the tail texture, which most of the animals have including Mouse.
The tail of an animal is an important body part, due to its vulnerability. It is recommended for all animals, even the largest ones, to keep their tails well-guarded, for the reasons below. Every animal except the Mouse, Shrimp, Chipmunk, Rabbit, Trout, Arctic Hare, and Crab should watch their tails, the exceptions due to the animals not having tails.
Tail-Biting is a game mechanic that allows an animal to fight back by biting the tail of a predator. If a tail is biteable, it will be highlighted with a light green outline. The only animals without a tail are the Rabbit, Trout. Arctic Hare, Mouse, Shrimp, Chipmunk, Snail, and Crab. Animals that are neutral (with a dark green outline) cannot have their tails bitten, unless they are tier 15 or above.
When it occurs - although uncommon if a predator is at least somewhat aware of their surroundings - it counts as one bite and the prey gets 40 XP + 2% of the bitten animal's XP, said to be with a maximum of double the biter's current XP, although there really is no maximum.
For example, assume a Rabbit that has 200 XP bites a Zebra that is less than 2K XP away from reaching Cheetah. When the rabbit bites it, the Zebra loses 400 XP(40+360(18000*0.02)), and the Rabbit immediately becomes a Fox, because it has 600 XP. The fox cannot bite tails for 3 seconds, due to upgrade invulnerability. If it were to bite 2 more times, then it would gain about 750 XP. Then, if it bites one more time, it immediately becomes a Zebra, about 1K away from Cheetah, When you Bite a Elephant,you get 10K xp from one bite from it's tail