Tail

Tails can be built using a variety of methods. This page is an index

Foam
Of course a tail can be modelled using foam and covered with fur. Sometimes the foam can be left empty inside to get a better swing.

Filling material
Cotton wool, Polyfill, and other fill material don't add to the structure but are often used because of pricing and availability. If you want the tail to have a special form you need to construct about the pattern accordingly.
 * example:Curly dog tail tutorial by Peking Snowfur

Metal
If other methods are not likely to produce good results, this is a risky method that might come in handy. Metal is very heavy and swings very well with the movements of the wearer, but it also increases the risk of injury around and of the wearer. Chains and wire are most likely to be used.

Connect
The conection to the wearer, most often through a belt or a harness. It's also possible to connect a very light tail directly to the body.

According to weight and material of the tail a piece of fabric, a hole in the foam, small magnets and various other methods can be used.