I heard somewhere that wild Tera Pokémon are always full odds, but what about non-tera spawns that are guarantied to spawn in the wild? An example would be the Belibolt on top of the mountain east of Alfornada.
No. These are called Fixed Pokemon Spawns, they can appear shiny but are not affected by any shiny Multipliers as discovered through data mining. Credits to Anubis for their efforts in data mining.
Hope this helps.