At the RuneHQ HM Mole event,
I knocked out a rotten fang.
When I clicked on it, Molly appeared!
Btw: As with most boss pet items, the Rotten fang follows the drop threshold system; it has a base drop rate of 1/2500 (1/500 in hard mode) and a threshold of 500.
When I was leaving after killing, and then restoring the Phoenix, for about the fifth time, I got lost and wound up in a chamber full of eggs (something like the Horta's).
I clapped my hands at one of the eggs, and this little fella appeared...