Nidoking --> Mega Nidoking
Type: Poison/Ground ---> Poison/Dragon
Ability: Poison Point | Rivalry | Sheer Force ---> Sheer Force
Stats: 81/102/77/85/75/85 BST 505 ---> 81/124/80/105/80/135 BST 605
New Moves: Dragon Claw, Draco Meteor, Dragon Rush (boosted by Sheer Force)
Concept:
Nidoking already has a dinosaur-like theme and access to Dragon moves like Outrage, Dragon Pulse, and Dragon Tail, so the new secondary Dragon-typing isn't too much of a stretch.
Since boosting moves are banned (no Dragon Dance), I decided to put 50 of the allowed 100 base stats in Speed (this may be a rule breaker, because I don't think there are any Mega Evos with this much of an increase in it's Speed stat.)
With 135 base speed, Mega Nidoking can effectively use either a Hone Claws sweeping set (Hone Claws | Dragon Rush | Poison Jab | Fire Blast/Earthquake) or an all-out-attacking set (Dragon Claw/Draco Meteor | Poison Jab | Earthquake | Fire Blast).