Let's say an ancient dragon, for some reason, fails a saving throw with legendary resistances left, but decides to save its legendary resistance and take the negative effect instead. Then, a round or two later, the dragon changes its mind, and decides it doesn't want this negative effect after all. Can the dragon at this point retroactively decide to use one of the remaining legendary resistances, in order to remove the negative effect? Or is that not allowed?
Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
fail: verb, 3rd person present
failed: verb, 3rd person past
The dragon fails a saving throw now, it can use Legendary Resistance. The dragon failed a saving throw then, it can't use Legendary Resistance.
Many ongoing effects allow saves after the initial saving throw - the dragon could use its legendary resistance on one of these contemporaneously. This would not work for spells that use subsequent ability check like Entangle because ability checks are not saving throws.