There are a couple that I discovered myself, which include:
Bounsweet nicknamed "Bouncee" from the Route 5 Pokémon Centre. It can be acquired before taking on the Water Trial on Akala Island, and it can be very helpful - it was a trade with an NPC for a Lillipup. It had an Adamant nature (but that could just have been random), had the ability Leaf Guard and held the Bright Powder item.
Aerodactyl - I don't remember when or where it was given to me, but I'm pretty sure that it was not a trade. It is level 40, has an Adamant nature (could be random) and the ability Rock Head.
Porygon - it is given to the player by an Aether Foundation Employee in the post-game inside one of the rooms of the Aeather House.
Type: Null - Gladion gives the player one in the post-game, on the top floor of the Aether Foundation, along with every memory disk required to change Silvally's type. Make sure to save before it though, so you can soft-reset to get the nature you want.
There is definitely more, and I would be interested in checking them out. If you discover any or actually have a complete list, please comment!
List of Pokemon you get as a Gift:
List of Pokemon you can get by ingame trading:
Source for both is the one and only Serebii.
