A fat drum is still a drum. This word can be used for any type of drum, but a common one in Japan is the taiko. There are lots of different types of taiko, and most of them are big and fat, and make a lovely deep sound. They're super cool to watch and play!

If talking about taiko drums specifically, you can also say 和太鼓 to be clear that you're not talking about any old drums.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.