The Brush (Pen)
The tip of the brush (毛筆) is made up of a bundle of animal hair attached to the end of a tubular cylindrical body, which is usually a bamboo stick:
Usually, the hair is obtained from wolf (狼), goat (羊), or horse(馬). Hair from other animals such as rabbit(兔) can also be used and some brush tips even offer a mixture of hair from different animals(兼毫).
The reason for the variation is because different animal hair offers different resilience, which in turn affects the result of the final product. For instance, I prefer the less elastic goat-hair brush when writing medium or large regular script style (中/大楷), or the clerical script (隸書), since the tip offers relatively more solid and firm appearance in your writing (Fig. 1 below). On the other hand, I prefer a wolf-hair brush when writing semi-cursive (行書) or cursive script styles (草書) since it offers a more fluid but strong appearance in the writing (Fig .2). Note that there is no absolute rule in matching the type of brush to a type of script style; it is all up to your tastes and preferences.