Assignments from the different professors of Hogwarts are highly useful as a means to learn lots of new and valuable spells. One of the assignments you get from Professor Howin will ask you to acquire a giant purple toad. If you are unsure how or where to get one, this guide will help you out.
At this point, you will have already obtained the ability to capture magical beasts using the case. This is what you will need to use here. You can find a giant purple toad den in the northern part of the map. Check out the video guide below for the exact location along with some tips on capturing the purple toad.