Adaptive Splice Rat

Agile. The rat doesn’t provoke Opportunity Attacks when it moves out of an enemy’s reach.

Adaptive Carapace. When the rat takes damage of a type it has not taken since the end of its last turn, it gains no special effect. If it takes the same damage type on two consecutive turns, its body rapidly restructures, gaining resistance to that damage type until it adapts again. This adaptation persists until the rat takes a different damage type.
The rat can only maintain resistance to one damage type at a time.

Adaptive Chromatic Shifting. The creature constantly shifts its coloration to match its surroundings using integrated chameleon and cephalopod traits. While in natural terrain (forest, swamp, ruins, mist): The creature has advantage on Dexterity (Stealth) checks. If the creature remains stationary at the end of its turn, it becomes visually difficult to track: Attacks against it are made with disadvantage until it moves or takes damage. Bright light, magical illumination, or direct observation from within 10 feet negates the disadvantage effect.

Bite. Melee Attack Roll: +2, reach 5 ft. Hit: 1 Piercing damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or take 1 necrotic damage at the start of its next turn as the wound irritates and refuses to close properly.