The brushes used to apply colour to woodblocks are a mixture of horse and pig or deer hair. They look like shoe brushes, as you’ll see from the photo below.
If the brushes are out of shape, I leave plenty of time for re-shaping as it can take all day.
Before use, the brushes are singed with fire, or a hot plate, and rubbed on a sharkskin, or modern metal equivalent. This splits the hair ends, and softens them, so that they hold more watercolour.