Is it reasonable to punish a paladin for breaking an oath to protect a creature they did not realize was a major antagonist?


Backstory

The events of my homebrew campaign take place after the fall of a powerful mage tyrant, assumed dead. About 6 sessions ago, my players recovered a living horse and dead human from a “manhunter spider”, used by bullywugs to track down persons by scent. They have come to adore the horse, naming it Wheatgrass and protecting it vigilantly. My paladin (will turn level 3 next session) just asked if they can make their Oath of the Ancients over Wheatgrass, swearing to protect this noble creature.

I had to restrain my excitement in agreeing to my paladin’s proposal, because of a key piece of information I’ve left out: the horse is, in fact, the mage tyrant in disguise. Depending on the narrative choices made by my players, they may soon discover that the spider was sent to hunt down the mage, though they still might suspect the dead human rather than the horse.

The magic used by the mage to transform into a horse form is protected by Arcanist’s Magic Aura (30 day casting), and my players have by misfortune avoided plot hooks that might have provided clues to the horse’s identity. To be fair, they have not been given a very good chance of discovering the horse’s identity until now.

Question Context

So when the Paladin turns level 3, they swear their Oath to the Ancients. The PHB states the following on oath breaking:

If a paladin willfully violates his or her oath and shows no sign of repentance, the consequences can be more serious. At the DM’s discretion, an impenitent paladin might be forced to abandon this class and adopt another, or perhaps to take the Oathbreaker paladin option that appears in the Dungeon Master’s Guide.

Protecting Wheatgrass is not one of the official tenets of the Oath of the Ancients, but my player proposes to include protecting Wheatgrass into this oath voluntarily. Now I am considering whether or not to impose a penalty in the event that the paladin attacks the mage tyrant. For example, if the mage tyrant revealed his true form, and the paladin attacked, perhaps the paladin would lose Oath feature until completing a nature-themed quest.

My Question

Are a Paladin’s class features contingent on upholding voluntary tennets their oath?

Would it be cruel or unreasonable of me to punish my paladin for unknowingly making an oath to protect a villain?

Considerations

As a GM, I know I can “do anything”; this is more of a should I question. I am giddy about the possible outcomes, but also worried that my excitement might mask the possibility of seriously upsetting a player.

Is there anything in the rules the explicitly forbids this? As a GM, I can overrule the rules, but this is more likely to upset players, who might want to have notice of such changes before choosing a class.

In favor of going forward, I would just add that there is something fittingly fey about this conundrum, which might work with the Oath of the Ancients.