Is the nine-tailed fox ability a triggered ability or a passive ability?

The Nine-Tailed Fox's ability is a triggered ability.

In Riftbound, triggered abilities are identified by the word "when" followed by a condition. Because the card text reads, "When an enemy unit attacks a battlefield you control, give it -1 might this turn," it functions as a triggered ability.

According to Rule 383, triggered abilities are repeatable effects that happen when a condition is met. Furthermore, as noted in the RiftJudge FAQ #3844, it is explicitly classified as a triggered ability (not a passive one) and must be announced by the player when the trigger condition is fulfilled.