1

The creature is merely a baby, and if it is defeated, its mother is going to be pissed.

2

The creature is actually a polymorphed adventurer with no control over their actions.

3

The creature is an infernal being from the pits of hell or some lower dimension of evil.

4

The creature is protected by some unholy power and whoever strikes the killing blow upon it will suffer some horrible curse.

5

The creature is a mad elder god unhappy with its current form. It will exchange its soul with the first character to strike it.

6

The creature is a gate to another plane, and its death opens the gate.