It doesn't work because Rarity relies on LibBabble-Boss, and it doesn't have an entry for this. Instead, it uses "Valiona and Theralion", which causes the defeat detection to always fail for the "Odd Twilight Egg" pet.

Until this is fixed, a workaround will have to be added. Thankfully, it's rather simple to do so, this issue being merely for future reference (as it may have to be removed later).

Fixed in 18d6b1f.