What stringed instrument is similar to a drum in that it has a goatskin or calfskin tightly stretched over one side of the head?

The instrument you're referring to is the tambourine. While it is primarily a percussion instrument, it often features a goatskin or calfskin head tightly stretched over one side, similar to a drum. The tambourine can produce both rhythmic sounds through shaking and striking, making it unique among stringed instruments. However, it's worth noting that traditional tambourines don't have strings; they are more closely related to percussion instruments.