There are multiple resources on the Internet for downloading and printing donkey mask templates. Sources such as Easy Child Crafts and Artists Helping Children provide templates and instructions for making donkey masks.
Artists Helping Children provides multiple links to donkey mask templates. These include printable paper mask templates, instructions for a paper mache donkey mask and a template for making a donkey mask using card stock. The site provides instructions for other donkey related kids' crafts, such as a piñata, paper bag puppets and clothespin donkeys. The required supplies vary depending on the mask.
Easy Child Crafts provides a template for a paper donkey mask as well as other farm and domestic animal masks. It recommends the use of masks for teaching children the names of animals.