Patch 2.3 changed the way Possess Bars work, but this change has not made its way to TBC Classic:
When you are possessing a target (Priest Mind Control, Eyes of the Beast, using a Steam Tonk, etc), you now get their action bar as your primary action bar rather than having a mini action bar above your normal action bar.
Source: https://wowpedia.fandom.com/wiki/Patch_2.3.0#User_Interface
Currently, we are still getting controlled unit’s actions on a pet bar, rather than as our primary action bar.
Other scenarios this applies to include (but are not limited to):
- Shartuul’s Transporter demons
- Teron Gorefiend’s Shadow of Death