A gift of the Catacombs

I encountered and tamed a dog after fighting most of the mobs in the Catacombs at Mine's End, but it keeps changing form. Could it be a sandestin? It’s already gone through all sorts of colors and shapes, including a dragon. If I use cancellation on it while it’s in... let's say dragon (or A-L) form, would it stay as a D (A-L) pet, or would it revert to its original sandestin form?