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 a skeleton horse.
Sometimes, Grim Reaper is depicted accompanied by a dog:
1. The Black Dog from the British folklore. The black dog is essentially a nocturnal apparition, often said to be associated with the Devil or a Hellhound. Its appearance was regarded as a portent of death. It is generally supposed to be larger than a normal dog, and often has large, glowing eyes.
2. Cerberus - the three-headed dog which guards the gates of the Underworld, to prevent those who have crossed the river Styx from ever escaping. Cerberus appears especially when Grim Reaper is depicted as a lord of the Underworld. Usually, in these tattoos Grim Reaper is depicted sitting on a throne decorated with skulls and human bones; Cerberus is sitting near the throne.
3. A new trend in the Grim Reaper tattoos is to replace the old-fashioned mythological dogs with Pit Bulls and Rottweilers.
Birds which are usually seen as bad omens that foretell of death like crows and ravens. Crows and ravens have been long time associated with death, sorcery, evil lords, haunted castles, etc.
Venomous snakes - the old symbol of pure evil is sometimes present in Grim reaper tattoos, but more often snakes are present in the skull tattoos (which have a slightly different meaning than the Grim reaper tattoos).
Increasingly popular some butterflies (especially the moths), bugs, spiders, etc. can occur in a Death tattoo.