Animal glue, also referred to as hot glue, can be found in various types. Most common are hide glue, rabbit skin glue, bone glue and fish glue. They are a by-product of the meat industry.
Glue must be applied to wood while hot, and forms a strong bond as it cools and loses moisture.

Animal glue is medium strong and keeps a certain flexibility. It has a very low moisture resistance, and the bond will weaken considerably when exposed to high humidity. Other downsides are the strong smell, and staining.

There are many positive features too. When heated, the glue is easily reversible. This is why old furniture and instruments made with animal glue can easily be repaired or restored. Furthermore the glue is easy to sand, and non-toxic.