Which type of seal pups were once killed for their white fur to line ski boots?

The type of seal pups that were once killed for their white fur to line ski boots are harp seal pups.

These pups, also known as "whitecoats," are born with a thick, white fur that is shed after a few weeks. This fur was highly prized for its warmth and softness, and it was used to line boots, coats, and other garments.

The commercial hunting of harp seal pups was widespread in the 20th century, but it has declined significantly in recent years due to international pressure and conservation efforts.