There are several animals which are often depicted in the Grim Reaper tattoo-designs. The most common of them is, by far, the Death's black horse. Despite the fact that initially (in the book of the Apocalypse) the Death is described as riding a pale horse, many tattoo artist choose to depict the Reaper riding a black horse (possibly due to the traditional interpretation of the color black representing famine, decay and death.) Another very common tattoo-motif is the Death riding ...
Source: Tattoo art.