Since megas got their own dex entries in Gen 7, some of them are pretty messed up. For example:
It tries to take the lives of anyone and everyone. It will even try to curse the Trainer who is its master! - Gengar, Ultra Sun
When the mother sees the back of her Mega-Evolved child, it makes her think of the day when her child will inevitably leave her. - Kangaskhan, Ultra Moon
When it Mega Evolves, it becomes more vicious than ever before. Some say that's because its excess of power is causing it pain. - Aerodactyl, Moon
It converts the energy from Mega Evolution into an intimidating aura. Fainthearted people expire from shock at the sight of it. - Absol, Ultra Moon
It readies itself to face its enemies by focusing its mental energies. Its fighting style can be summed up in a single word: heartless. - Lucario, Moon
Mega Evolution fuels its brutality, and it may even turn on the Trainer who raised it. It's been dubbed "the blood-soaked crescent." - Salamence, Moon
Its intellect surpasses its previous level, resulting in battles so cruel, they'll make you want to cover your eyes. - Metagross, Ultra Sun
Also, regular Gengar in Sun:
Should you feel yourself attacked by a sudden chill, it is evidence of an approaching Gengar. There is no escaping it. Give up.