I think it’s critter specific. Imps aren’t known for shedding their horns so they are permanent even in death, especially since so much of the head is actually horn. All the goat types (Satyr, Geitlan) don’t shed. But dear types like Wendigo will shed their antlers in life so those are going to drop off in death.

The one I’m trying to figure out is kobold skull. We are used to seeing dragon skulls with their horns in tact but I don’t know if that’s a trick of taxidermy or not. And kobolds aren’t full dragon, they’re just a little dragon. Might need to go underground and do some research on that.