In my canon, the icy structure on Glaceon's forehead is actually a permafrost helmet meant for headbutting like a ram. They get it when they evolve but after a while, the ice may get worn and damaged and eventually break. A Glaceon doesn't need its helmet to survive, as certain individuals do quite alright without them. In the wild though is when the permafrost helmet is mandatory and crucial for survival.

Trainers. If your Glaceon's helmet is broken, you can go to a store and buy a helmet mold. Similar to how ice trays work, just pour water into the mold and put it in the freezer for a while. When the new helmet is ready, your Glaceon may make some modifications to it for extra durability before licking it and sticking it to their forehead. And just like that, your Glaceon is good to go with an easy-to-make replaceable ice helmet!