Alexander, the unlucky puppet king of Greece, was killed by a monkey bite and medical incompetence in 1920.

Alexander, the second son of Constantine I of Greece, was not a lucky guy. Being the second son of a king is rarely lucky (unless you’re a fan of fratricide), but Alexander’s life was especially unlucky.
It all began with Eleftherios Venizelos. Venizelos was the prime minister of Greece for fifteen years, including a stretch over World War I. Under his watch, Greece’s territory doubled: it united with Crete and liberated Macedonia, Epirus, and many of the islands in the Aegean from the Ottoman Empire. But Venizelos also came into conflict with Alexander’s father, the king.
The prime minister wanted to ally with the Entente powers in World War I – Russia, France, and Britain – but the king wanted to remain neutral. King Constantine had in fact secretly promised neutrality to Germany and Austria-Hungary. The political dispute between the prime minister and king is today known as the National Schism. It got messy.
The king forced the prime minister to resign… twice. The prime minister got the king to abdicate… twice. (Well, the second time was forced by pro-Venizelos army officers due to a bad loss in the war against Turkey, but close enough.) The first time King Constantine abdicated, his eldest son did not become the new king. George, also pro-neutrality, followed his father into exile… so Alexander became king.
(George eventually did become king too, although he went into exile a further two times over the course of the 20th century.)
Alexander was a puppet king. Trapped in the palace and surrounded by anti-royalist followers of Venizelos, he sent letters to his exiled family and waited. Those letters, of course, never reached them. About the only thing he could do was secretly marry his childhood sweetheart – and that took three attempts.
(His wife, Aspasia Manos, was not royalty. The marriage was intensely controversial and technically illegal because they had not received the archbishop’s permission first.)
Cut to 1920. Alexander was walking his dogs in the palace grounds when a Barbary macaque attacked. It attacked the dog, actually. Alexander tried to separate the two animals. Another macaque – presumably a friend of the first – joined the fight. It bit Alexander on the body and leg then ran off. Doctors cleaned the wounds, but not well enough.
The leg became infected, and that infection spread. He might have been saved by amputation, but no-one wanted to be responsible for removing the king’s leg. Alexander died twenty-three days after the monkey bite. None of his family were with him when he died.
As for the throne, it remained vacant until Constantine returned from exile and Venizelos went into exile – so Alexander both succeeded and was succeeded by his father. The Greek royal family didn’t officially recognise Alexander’s reign, sandwiched as it was between Constantine’s two reigns, although they did posthumously legitimize his marriage.