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

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Charge. If the beast dragon moves at least 20ft straight towards a target and then makes a gore attack as a bonus action. 

Alternate form. The beast dragon can transform into a beast or monstrosity of any cr lower than its own. The beast dragon gains all the statistics of the creature except for their intelligence, wisdom and charisma scores and proficiencies. When they are reduced to 0 hit points or incapacitated in the alternate form they revert back their true form but on half hit points. They can go in and out of this form as an action.

Innate Spellcasting. The adult beast dragon's innate spellcasting ability is Wisdom. The adult beast dragon can innately cast the following spells, requiring no material components:

3/day: jump

Multiattack. The adult beast dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage plus 7 (3d4) cold damage and 7 (3d4) lightning damage. 

Gore. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (1d12 + 7) piercing damage and the target must make a DC 21 Strength saving throw or be pushed 15ft and knocked prone. 

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 7) slashing damage.

Tail. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 18 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Lightning blizzard breath (Recharges 5/6). The beast dragon exhales a blizzard like cloud which crackles with lightning, each creature within a 60ft cone must make a DC 19 Constitution saving throw or take 27 (6d8) lightning damage plus 27 (11d4) cold damage. On a successful saving throw a creature takes half damage. 

The adult beast dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The adult beast dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Bashing roar (Costs 2 Actions). The dragon slams its horns into the ground roaring loudly whilst uplifting the ground around a 10ft radius square right infront of them. Each creature within that radius must make a DC 21 Strength saving throw or be pushed 15ft and knocked prone and they also must make a DC 19 Constitution saving throw or be deafened for 1d4 minutes.