Unfortunately, your options are limited and many will require DM approval that is outside RAW.
A deep dip (5 levels) into Paladin or Bard is the only way to get the 2nd level spell Find Steed - which is exactly what you're looking for here (I think).
However, there are magic items that do the Druid bit of talking/being friendly with animals - but that doesn't really give you the control for attacking that you're looking for. However, a combination of the item and your DM may be enough:
- Ring Animal Influence (rare, DMG 189) - Cast Animal Friendship, speak with animals
- Bag of Tricks, Gray (uncommon, DMG 154) - 1/8 chance of getting a panther (maybe have the DM alter this?)
- Ring of Spell Storing (rare, DMG 192) - 5 Find Steed Spells (if your DM is nice)
Also, remember that a Panther has an AC of 12 and 13 HP. That's not gonna stand up long. You'll likely need custom barding (if you can even get it to wear it!) to boost that. But any natural panther (if your DM let you find/train one), won't last long.
The Bag of Tricks may be your best bet. Maybe your DM will change the animals or make it just a Bag of Panthers.