Whistl

"???" Elements: Shadow, Mech, Crystal, Poison, Spectral

This monster can be bred with a Double Element and a Triple Element.

Frogget + Boodoo

Triochoir + Drummech

Rillaquina + Kazilleon

Skulltar + Flutehog

Sitarthon + Fung Pray

Didgetiki + Bellowfish

Gockonspiel + Googlug

Jirobeat + Arackulele

Stong + Dragong

Pipgan + Shadowmorph

However, it can be bred with a Single Element and a Quad Element.

Monosynth + Grumpyre

DooDa + Reebro

Polkacloud + Jeeode

Spacehorn + Humbug

C-Xbot + Spectgon