Just for those wondering, this is the definition of headcanon.
-Lucario's yellow tank top and chest spike, and shorts are removable, but the black ringlets around its shoulders are literally its shoulders.
-Machamp grew an extra pair of arms during evolution out of sheer willpower to become stronger when it was a Machoke.
-Mimikyu is just a Shuppet who admires Pikachu so much it cosplays as it, however, the costume is too heavy for it to maintain levitation, so it becomes grounded.
-Decidueye used to be Grass/Flying, but went extinct when humans began migrating to Alola. Using the same technology used to revive fossils, scientists revived the Decidueye, however it became Ghost in the process.