Animals are a popular ESL topic especially with young learners and students in primary school. Here at BusyTeacher there are 555 worksheets in this section that focus on animals as well as a variety of grammar points. These are all ready to use, free, and printable which makes the site a great resource for teaching materials. Take a look at this worksheet which combines animal vocabulary and reading practice into a guessing game for students which they can complete individually, in pairs, or even in groups. If you would like to do something else, look at some other worksheets to find something your students will love.
Animals are a fun topic that students enjoy talking about so they can be used throughout your course to interest students in a variety of activities. Younger students especially like talking about different types of animals so be sure to give them the opportunity to use this enthusiasm in their ESL classes. Besides studying animal vocabulary, you can also use animal flashcards when studying comparative and superlative structures, descriptive words and for other topics too. With intermediate, advanced, and adult learners animals will be a less important topic but you may consider talking about some of them in discussions, for example polar bears and global warming.