I was looking around googling about that. It's really easy to do, I just did a performance for my Observation pet, it looks weird because I don't really like to make animations and I have to use the sprites on .spr file of course.
Anyway, it's pretty easy to do if we can gather a "Sprite team" to draw the performance sprites and I teach everyone how to put it on the pet!
If you want to give this a try it could be interesting, the monster sprite itself has to be modified to include an animation, act file might need changing too to specify from which frame to which. Anyhow, not something I would wanna do myself, but if it's provided could be a welcomed addition to replace the disappearing act that custom monster currently do.
It's really easy, someone just need to make the monster using whatever you'd want as perfomance and adding the sprites to the monster's .spr file. I'd make the .act, easily.
You could even use a mixture of the mosters already existant animations...
Maybe a pet will take a swipe, then sit suddenly, and do the confused emote, or some such?
Yeah, I was thinking of making them do like the attack animation or something. Can't make them do emotes though... (not from .spr/.act)
But nothing better than totally custom performances for heRO custom pets made by the players!