As far as I recall, the “Bag of Tricks” mod can do this, as animal companions are definitely missing from the bottom bar when the inventory is not open.
The problem was that this added the “scroll left/right” arrows you normally see when inventory/spellbook is open. They then overlapped with the “autocast” on the left and the “toggle ai/stealth” on the right. This became especially annoying because AI always resets itself to on when leaving turnbased mode (which I consider a bug).
The “bag of tricks” mod, however, also allows adding a 2nd healthbar to every party member that has an animal companion. Which is what I ended up using.
PS: I might have been wrong, it could have also been the mod “UI Extensions”.
PPS: Yes, here it is:
And here is the unfortunate overlapping: