This is probably the rarest collectible in this set. Did this HA 9x and they dropped as follows:
1 Undead Spider
5 Inedible Goo
5 Reanimated Wall Brick
6 Twitching Bone Chips
5 Bristling Animal Pelt
1 Fluttering Heart
3 Chattering Teeth
6 Wriggling Antennae
4x I didn't find a collectible
Undead Spider is rare too but I did see this one offered for sale in the Bazaar. Never happened with Fluttering Heart.