Mega Leavanny are a quite rare sight to see, and even harder to find. Most Leavanny refuse to ever Mega Evolve, because while the gained power is undeniable, the threat of harming those they care about is too high price of a cost to pay for the caring Pokémon such as them. The ones who choose to Mega Evolve despite this usually are desperate, or have no-one left to loose. Or, perhaps they've already gone mad, so there is no further harm in it.

Commonly Mega Evolving will cause the Leavanny to become ostracized from its wider family and community, as they are now seen as a danger, rather than an ally. Many families tell their young Sewaddles and Swadloons horror stories of things Mega Leavanny in their distant family might have done at some point, to make sure they stay far away from all of Mega Evolution altogether. 

Most Mega Leavanny are solitary, but occasionally they form their own found family groups, desperate for some sort of connection that won't cast them away despite their inner madness slowly consuming their self-control. It's not uncommon that Mega Leavanny might end up stuck in their Mega form for extended periods of time, unable to return to their base form unless fainted, due to this slowly growing madness within them.

Evolution

Mega Evolves from Leavanny after being exposed to Leavannite.

Base Stats

HP: 75
Attack: 133
Defense: 80
Sp. Atk: 70
Sp. Def: 100
Speed: 142

Total: 600

Type Effectiveness

Weak to (2x): Flying, Rock, Fire
Damaged normally by (x1): Normal, Poison, Bug, Ghost, Steel, Water, Electric, Psychic, Ice, Dragon, Dark, Fairy
Resistant to (½×):  Fighting, Ground, Grass