Great idea! Just finished DMing. Exhausted. Will sleep on this but inspired...
Came up with this (lvl 3 exhaustion captures an distracted and part-bewildered state well...and is easy click on Dndbeyond etc.). Also ability is consistent with but distinct from jackalwere eye-gaze-sleep and wolfwere music-slow 9wasn't sure about range....30 ft? 60 ft?)
War Dance. As an action (requiring Concentration), a mongoosewere cavorts and prances in such a manner that distracts and bewilders opponents, who must make a DC 12 WIS save or suffer the equivalent of three levels of exhaustion. A successful roll means the subject is immune to the war dance effects for one hour. A subject can re-attempt an unsuccessful roll at the end of subsequent turns in combat.
Hmm, but wouldn't that mean that if they're exhausted from another source they might collapse or pass out?
It seems prudent to add a proviso that this pseudo-exhaustion does not stack with actual exhaustion.
Also, Fatigue Levels are an optional rule and Disadvantage on ALL saving throws plus a -3 penalty seems rather extreme. Why would CON saves be disadvantaged for being distracted? I'd suggest limiting it to, say, DEX and INT saves, perhaps with penalties to Initiative?
Heck, it seems easier to just state the penalties in the text, maybe with just simplifying it a bit to just Disadvantage and a few minor penalties.
War Dance also doesn't have a duration and the "save each round" doesn't follow the usual wording.
So that'd be something like this:
War Dance. As an action (requiring Concentration), a mongoosewere cavorts and prances in such a manner it distracts and bewilders opponents, who must make a DC 12 WIS save or become mesmerised for [duration]. Mesmerised creature suffer disadvantage on [STUFF], move at half speed [and cannot perform reactions?]. A mesmerised target can repeat the saving throw at the end of each of its turns. If the saving throw is successful, the effect ends on it. A target that successfully saves is immune to this mongoosewere's war dance for the next hour.[/
]
However, I was thinking more that the War Dance targets a specific creature close to the Mongoosewere who the monster "prances in front of" and it only lasts as long as it's dancing plus maybe a round or two after it stops. If you like that idea, I'll have to think about how to word that.