Stronger

Stronger (ストロンガー) is the name of the horse that Doc Q rides on. Because Doc Q is seemingly always sick, he rides on Stronger all the time, since the doctor tends to collapse if left unsupported for some time. However, Stronger is just as sick as he is.

Abilities and Powers
Stronger is a very sick horse, so it is a surprise that it could still have the strength to carry Doc Q around. But it's abilities might not have been fully shown, as it is doubtful if Blackbeard would ever recruit a weakling into his crew.

Mock Town
Stronger is simply shown to carry his rider, Doc Q, throughout their appearances in Mock Town.

Banaro Island
Stronger was once again carrying his sick master during the raid on the Blackbeard Pirates' Banaro Island. When Portgas D. Ace attacked them, Stronger and Doc Q fell while running to avoid their captain's retaliation, and was picked up by fellow crew members Auger and Burgess respectively. They all watched from a safe distance as Blackbeard defeated Ace.

Impel Down
During the Blackbeard Pirates' attack on the Great Jail: Impel Down, Stronger is once again carrying the sickly Doc Q. After an exchange of words between Blackbeard and Monkey D. Luffy, Stronger was poisoned along with the rest of the crew by Magellan's Hydra technique. Later, the whole crew is saved by an antidote provided by former Chief Guard Shiliew, who accepts Blackbeard's offer and joins their crew.

Marineford
Stronger and his owner Doc Q are spotted along with the other members of the Blackbeard Pirates on the execution platform where they were watching the events that unfold on the battlefield. Currently he and the others came to Marineford because they wanted to witness Whitebeard's death.

After Whitebeard dies, Stronger and his master assist their fellow crewmates in covering Whitebeard's body in a black cloth and guard Blackbeard as he begins to demonstrate his powers. Stronger, Doc Q and the other Blackbeard Pirates are then attacked by a shock wave created by Sengoku in his Buddha form. When Shanks arrived, Stronger leaves with the rest of the crew.

Post War Arc
Stronger and the Blackbeard Pirates make their move in the New World and take out Jewelry Bonney and her crew. He is with Doc Q when the doctor talks about heaven's role in fate. Later, when Akainu shows up, Stronger is seen carrying Doc Q as the two flee with the rest of the crew.

Trivia

 * It is ironic that Stronger is sick, because horses are generally portrayed as an image of good health.
 * With Doc Q nickname "Shinigami", the sick nature of the horse may be a reference to Pestilence, one of the Four Horseman of the Apocalypse, a horse known to cause sickness.