Drinking from an animal horn

Hi rabbi I was looking to purchase a beer mug made out of an animal horn. Does this pose any halachic issues. If no can you use it with milk or with hot liquid. And would I have to dip this cup?

Koshering

When you dip silverware into a boiling pot and take it out do I need to run it under cold water after?

Kosher fish

Is it ok to buy fish that is frozen and individually packaged ( like in Costco) even if it is skinless