You take a Pokemon without a Mega, give him a Mega-Stone, a new ability, typing, stats and some moves in case they apply to give the mon a niche. They need to somewhat fit the mon.

I will start

Mega-Skuntank
Dark/Poison
103 HP
93 Atk --> 63 Atk
67 Def --> 117 Def
71 SpAtk ---> 101 SpAtk
61 SpDef ---> 111 SpDef
84 Spe ---> 84 Spe

New Ability: Poison Gas
Poisons the opponent if attacked by any contact-move.

Gains Venoshock, Sludge Wave, Slack Off
Makes Dark Pulse usable and the loss of Sucker Punch doesn't hurt with Slack Off