blue dread rat

Keen Smell. The rat has advantage on Wisdom (Perception) checks that rely on smell.

Pack Tactics. The rat has advantage on an attack roll against a creature if at least one of the rat's allies is within 5 feet of the creature and the ally isn't incapacitated.

lightning fur. anything within 5ft of the dread rat takes hit:3 (1d6) lightning damage

four leg run. the dread rat runs at double speed but has to drop its weapons

shrink. the dead rat can shrink to any size smaller than its true form

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7(1d8 + 4) piercing damage. and hit:3(1d6) lightning damage and increasing by hit:2(1d4) lightning damage per 8 health left of target 

claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7(1d6 + 4) slashing damage. and hit:3(1d4) lightning damage and increasing by hit:2(1d4) lightning damage per 8 health left of target

tail. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 7(1d6 + 4) bludgeoning damage. 

lightning breath (Recharge 2-6). the rat exhale  lightning in a 15ft cone each creature in that are must make a DC 14 dexterity throw or taking 20 (5d6) lightning damage or half as much on a successful save