Adaptive Form. The Zamtrios has access to two alternate forms, each with their own advantages in battle.

Ice Armour (Recharge 6). When enraged, the Zamtrios can encase its form in durable, jagged ice as an action. This ice armour lasts for 1d6+2 rounds, or until the Zamtrios takes fire damage equal to or greater than 50 points in one round.
While encased, the Zamtrios' melee attacks deal an additional 2d8 cold damage, and its AC increases by 2.

Inflatable Hide: The Zamtrios possesses a unique physiology which allows it to inflate itself to gigantic proportions in response to heavy injuries. If an attack drops the Zamtrios' HP to half or less, the Zamtrios can use its reaction to inflate itself. The Zamtrios gains 3d10+10 temporary hit points and resistance to bludgeoning, piercing and slashing damage, but its AC drops by 2, its walking speed becomes 10 ft., and its swimming and burrow speeds become 0 as it becomes an easier, cumbersome target. In return, the Zamtrios gains access to several unique abilities while in this form, which replace its list of actions when normal or encased in ice armour. 
This form lasts for 1d6+2 rounds, and cannot be used again until the Zamtrios finishes a short or long rest. When the Zamtrios exits this form, creatures within 20 ft. must succeed a DC 15 Constitution Saving Throw or take 11 (3d6+2) cold damage and have their movement speed halved until the start of the Zamtrios' next turn, as a freezing gas is expelled.

Inflatable Hide and Ice Armour cannot be active simultaneously. If the Zamtrios chooses to inflate while ice armour is active the ice armour is broken, and creatures within 20 ft. of the Zamtrios must succeed a DC 15 Dexterity Saving Throw or take 3d6+2 cold damage as ice splinters shower the area.

Normal and Ice Armour Forms

Tail Slash. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 9 (1d10 + 4) slashing damage (+5 (2d8) cold damage when encased in ice armour).

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 8 (2d6+2) slashing damage (+5 (2d8) cold damage when encased in ice armour).

Charge. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. The Zamtrios moves up to its walking speed in a straight line toward a creature it can see. Any creatures between the Zamtrios and the target during its charge must succeed a DC 13 Dexterity Saving Throw or take half the damage of the target of the charge. Hit: 19 (3d10+4) bludgeoning damage (+5 (2d8) cold damage when encased in ice armour), and the target must succeed a DC 15 Dexterity Saving Throw or be thrown 20 ft. away and knocked prone.

Multiattack. The Zamtrios makes three attacks, one with its tail and two with its claws. It can alternatively replace two of these attacks with one use of its charge.

Cold Spit. Ranged Weapon Attack: +7 to hit, range 30/120 ft., one target. Hit: 11 (3d6 + 2) cold damage, and the target must succeed a DC 14 Constitution Saving Throw or have its movement speed halved until the beginning of the Zamtrios' next turn.

Inflated Form

Leaping Crush. Melee Weapon Attack: +7 to hit, reach 30 ft., one target. The Zamtrios leaps into the target's space, dealing 19 (3d10+4) bludgeoning damage and the target must make a DC 15 Strength Saving Throw or be pinned by the Zamtrios.

Rolling Crush. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. The Zamtrios moves up to its walking speed in a straight line toward a creature it can see. Any creatures between the Zamtrios and the target during its charge must succeed a DC 14 Dexterity Saving Throw or take half the damage of the target of the charge and be knocked prone. Hit: 24 (5d10+4) bludgeoning damage, and the target must succeed a DC 16 Dexterity Saving Throw or be knocked prone.

Mass Cold Spit. Ranged Weapon Attack: +7 to hit, range 20/60 ft., three targets. Hit: 11 (3d6 + 2) cold damage, and the target must succeed a DC 14 Constitution Saving Throw or have its movement speed halved until the beginning of the Zamtrios' next turn.