Often black legs simply look like a pony has been dipped into ink on their legs. They can be on all four legs or just the front two.
There are a couple of stripe patterns for black.
Tall socks go up to a pony's elbows in front and knee in back. This is distinct from the white underbelly (gradient) marking which continues up a pony's flank.
Medium socks go up to the knee in front and hock in back.
As the name suggests, these go just to the ankle.
Black + WhiteEdit
Note, a pony can inherit both black and white socks.
Other leg colours are also possible. The Blue Legs were originally seen on the Undead Mare from Halloween 2012 while the Two-Toned Legs were added recently.