Xenacanthus

Blood Frenzy. The xenacanthus has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Electrocyte. The xenacanthus possesses an electric organ. A creature that touches the xenacanthus or hits it with a melee attack while within 5 feet of it takes 3 (1d6) lightning damage, and the creature must succeed on a DC 12 Constitution saving throw or be stunned for 1 minute.  A creature stunned by this trait can repeat this saving throw at the end of each of its turns, ending the effect on itself on a success.  This organ itself is not magical.

Hold Breath. While out of water, the xenacanthus can hold its breath for 1 hour.

Venomous Spine. The xenacanthus has a venomous spine protruding from its skull. Whenever a creature touches the xenacanthus or hits it with a melee attack while within 5 feet, roll a d6.  On a 1 to 5, the attack occurs normally.  On a 6, the attacker takes 3 (1d6) piercing damage plus 4 (1d8) poison damage and must succeed on a DC 12 Constitution saving throw or be poisoned until it finishes a long rest.

Water Breathing. The xenacanthus can breathe only underwater.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage.

Jolt. Melee Spell Attack: +4 to hit, reach 5 ft., one target. Hit: 3 (1d6) lightning damage, and the target must succeed on a DC 12 Constitution saving throw or be stunned for 1 minute.  A creature stunned by this action can repeat this saving throw at the end of each of its turns, ending the effect on itself on a success.