An animation only shows on an entity whose model has the bones it moves, so pick one made for the entity you target.
Loading the animations...
The animations come from the 143 animation files in the Bedrock 1.26.52 resource pack, so the list holds exactly what the game can play, from a player sleeping to a warden roaring. Every one of them was played on a simulated player on a Bedrock Dedicated Server, along with each mix of the optional arguments, and every command succeeded.
The controller name decides how animations combine. Two commands with the same controller replace each other, while different controllers run side by side, so a wave and a nod can play at once on the same entity.
The entity's model needs the bones the animation moves, and the command reports success either way: on a test server, every one of the 539 animations, pig and warden ones included, succeeded on a player. So pick an animation from the entity you target. The list is grouped by the file each animation comes from, which is usually the entity it belongs to.
Give it a stop condition that stays false while you want it running, like query.is_sneaking to play until the player sneaks. Leave it empty and the animation stops when it finishes.
/playanimation reads its extras in order: next state, blend out, stop condition, controller. Setting the controller alone means the three before it have to be written too, so the generator fills them with their defaults: default, 0 and query.any_animation_finished.
