These are all the guys.

Red is regional, white is evolution, blue is mega.

No type means no change.

Xerneas and Yveltal forms function like Megas but aren't, they're called Eternal Xerneas and Eternal Yveltal.

Zygarde receives no new forms, but the move Core Enforcer is changed to have 5 pp, and activates Complete Form regardless of HP.

Onix -Frozen Blend item-> Chillix (Ice/Ground)
Shuckle -Tangle Slap move-> Untuckle (Bug/Rock)
Wimpod -Lvl 30-> Evasopod (Bug/Dark)
Thievul -Lvl 40-> Infernul (Dark/Fire)
Boltund -Lvl 42-> Frayhound (Electric/Fairy)

Chesnaught / Delphox / Greninja -> Paladin / Mage / Knight
Diggersby -> like annihilape
Pyroar -> circus performers
Gogoat -> wild ram
Cloyster -> spike trap
Hypno -> nightmare
Crobat -> gargoyle
Slowking -> inverse slowbro
Flygon -> reverts to prevos
Milotic -> nessie
Chimecho -> dreamcatcher of windchimes
Excadril -> bulldozer
Sigilyph -> tapestry
Maractus -> flaming cactus
Haxorus -> made of axes
Cofagrigus -> pyramid
Volcanion -> steam engine
Hoopa -> middle eastern genie
Xerneas -> majestic stag, monochrome blue
Yveltal -> evil condor, monochrome red