Bowser

Legendary Resistance (3/Day, or 4/Day in Lair). If Bowser fails a saving throw, he can choose to succeed instead.

King of Koopas. Bowser can attack twice per action, and can take two actions per turn. Spells in the Spellcasting list take a full action as normal.

Magic Resistance. Bowser has Advantage on saving throws against spells and other magical effects.

Unnatural Existence. Alongside Dragon, Bowser also counts as a Monstrosity.

Dry Bowser. If Bowser is submerged in lava, either by his own action or another creature's, he is incapacitated for a turn, before transforming into Dry Bowser. He retains his current health and used actions, but all conditions and debuffs are removed.

Strong Lungs. Bowser can hold his breathe for up to one hour, and can resist certain gas based attacks and affects for as long as air remains in his lungs.

Powerful Build. Bowser counts as two sizes larger when determining his carrying capacity and the weight he can push, drag, or lift.

Teleport. Bowser can freely cast Teleport outside of encounters.

Punch. Melee Attack Roll: +10, reach 10 ft. Hit: 20 (4d8 + 2) Bludgeoning damage.

Fire Shot. Spell Attack Roll: +10, reach 60 ft. Hit: 20 (4d8 + 2) Fire damage. 

Fire Breath (Recharge 5–6). Dexterity Saving Throw: DC 19, each creature in a 60-foot Cone. Failure: 54 (12d8) Fire damage. Success: Half damage.

Bowser Bomb (Recharge 5–6). Dexterity Saving Throw: DC 19, each creature in a 15-foot Sphere. Failure: 45 (10d8) Bludgeoning damage, knocked Prone. Success: Only half damage.

Spellcasting.  Bowser casts one of the following spells, requiring no Material components and using Intelligence as the spellcasting ability (spell save DC 19):

At Will: See Invisibility, Burning Hands (level 2 ), Melf's Minute Meteors (level 3)
1/Day Each: Lighting Bolt (level 6), Call Lightning (level 6, doesn't require free space to cast), Chain Lightning, (level 6), Enlarge/Reduce (level 2), Polymorph (level 4)
1/Day, Mythic Only: Meteor Swarm,  Bones of the Earth (level 6)

Opportunity Attack. Melee Attack Roll: +10, reach 5 ft. Hit: 14 (3d8) Bludgeoning damage.

Withdraw. As a reaction to an incoming attack, Bowser can withdraw into his shell. While in his shell, Bowser gains the benefits of the Shield spell (+5 AC and immunity to Magic Missile). Additionally, any unarmed melee attack made at Bowser while Withdraw is active will cause the attacker to take the damage of the attack they just made, translated into Piercing damage, + 2d6 Piercing damage. This lasts until the start of Bowser's turn.

Grow. Bowser can grow to Gargantuan size. This state lasts until the end of the battle. He gains temporary health equal to his default maximum health and his Strength and Constitution scores are doubled. The range and damage on all of his actions also double, and he gains advantage on melee attack rolls.

While in this state, Bowser's Dexterity score is halved and he loses access to reactions. Furthermore, if lair actions and effects were active during the fight, they have been disabled unless the DM rules otherwise.

If there are any buildings or similar objects obstructing Bowser's growth, they are destroyed unless the DM rules otherwise.