A few Pokemon in S/V have flying (or/and swimming) variations of their idle animations when in (and out of) battle, some Pokemon have it as their default animation (most bird Pokemon have flying as their default (yes they also all have grounded idle animations now but only out of battle)), when swapping Pokemon and sending out another one if the Pokemon you switched out was using its flying in-battle idle animation and the Pokemon that switched in has a flying variant of its in-battle idle animation (even if its default idle animation is on the ground (like decidueye)) the Pokemon you switched in will use the flying variant of its idle animation instead
Example: A battle starts, you have a Staraptor on the field, Staraptor’s default idle animation in battle is its flying idle animation, you switch out Staraptor for a Decidueye, a Pokemon whose default idle animation is its grounded idle animation, Decidueye also has a flying idle animation even though it’s not its default idle animation and because the Pokemon (Staraptor) you switched out for Decidueye was using its flying idle animation Decidueye will also use its flying idle animation if it has one (which it has)
Source: Sadly none, But you can easily check in game if you don’t believe me
(sorry if this is hard to read, I tried my best to explain this without using complicated terms which made it a lot harder to explain)
