Shapechanger. The wereturtle can use its action to polymorph into a turtle-humanoid hybrid or into a Medium turtle, or back into its true form, which is humanoid. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Hold Breath. The wereturtle can hold its breath for 1 hour.

Innate Spellcasting (Humanoid or Hybrid Form Only). The wereturtle's innate spellcasting ability is Wisdom (spell save DC 12). The wereturtle can innately cast the following spells, requiring no material components:

At will: druidcraft, poison spray, resistance
2/day each: create or destroy water, cure wounds, entangle, pass without trace, spike growth
1/day: plant growth

Stable (Turtle Form Only). Whenever an effect knocks the wereturtle prone, it can make a DC 10 Constitution saving throw to avoid being knocked prone. A prone wereturtle is upside down. To stand up, it must succeed on a DC 10 Dexterity check on its turn and then use all its movement for that turn.

Underbelly (Turtle or Hybrid Form Only). While the wereturtle is prone, the AC from its natural armor is reduced by 4.

Bite (Turtle or Hybrid Form Only). Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) piercing damage. If the target is a humanoid, it must succeed on a DC 12 Constitution saving throw or be cursed with wereturtle lycanthropy.

Slam (Humanoid or Hybrid Form Only). Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 1) bludgeoning damage.

Sling (Humanoid or Hybrid Form Only). Ranged Weapon Attack: +3 to hit, range 30/120 ft., one target. Hit: 3 (1d4 + 1) bludgeoning damage.

Poison Spray (Cantrip, Humanoid or Hybrid Form Only). One creature that the wereturtle can see within 10 feet of it must succeed on a DC 12 Constitution saving throw or else take 13 (2d12) poison damage.

Withdraw (Turtle or Hybrid Form Only). The wereturtle withdraws into its shell. Until it emerges, it gains a +4 bonus to AC and it has advantage on Strength and Constitution saving throws. While the wereturtle is in its shell, its speed is 0 and can't increase, it has disadvantage on Dexterity saving throws, it can't take reactions, and the only action it can take is a bonus action to emerge from its shell.