Fortuna Mala is an aspect of Fortuna, the Roman Goddess of luck and chance Who is able to ward off bad luck, because She can also bring it. Fortuna in general was considered beneficial, but also capable of being inconstant and fickle; as She sometimes rewarded those who plainly did not deserve it with excellent good luck, She was occasionally portrayed as blind. Fortuna Mala is Fortune at Her worst; She causes the unlucky series of events common to tragedies, and is that Fortune Who, according to Shakespeare "plays the huswife", or the fickle hussy (which my old dictionary describes as "a pert, frolicsome wench").
The Romans, in their superstitious mindset, may have thought that Fortuna Mala was a force of balance in the Universe, in a Murphy's Law sort of way, rather like the modern aphorism "No good deed goes unpunished". Which is sad; but there are plenty of examples of this kind of philosophy in Roman history—for instance victorious Roman generals were sometimes celebrated in great processions called triumphs in which the spoils of victory were displayed; but it was also traditional that a slave stood behind the general in his chariot and constantly whispered, Respice post te, hominem memento te ("Look behind you! Remember that you are just a man!") or Memento mori, ("Remember you will die").
This attitude is illustrated in an episode of the life of Aemilius, a Roman general of the 2nd and 3rd centuries BCE. He had sacked Epirus, a region of northern Greece, and was granted a triumph on his return home. But his grand moment came between the deaths of two of his sons, one of whom died five days before, the other just three days after the triumph. Aemilius is said to have regarded this as a balancing force of evil the Gods sent him to counteract the good fortune he had experienced in battle.
Fortuna Mala had a very old altar on the Esquiline Hill, though where exactly is not known. She is also said to have had a temple somewhere in Rome, as did Her alter ego Fortuna Bona, "Good Fortune"; whether it is connected with the altar on the Esquiline is unknown. Cicero, a philosopher, statesman, and lawyer of the time of Julius Caesar, complained that Fortuna Mala's altar on the Esquiline should have been abolished, as should the altar to Febris, or "Fever", as they celebrated vices rather than virtues; I think in his "modern, rational" view he was missing the point that such forces not only act to balance things, but as dark forces they especially need to be honored and placated, so they will see fit to not bring their misfortunes upon the people.
Fortuna Mala was depicted on coins, as was Fortuna Bona.
She was also called Mala Fortuna.
She is also sometimes called "Fortune With a Beard"; I don't know where this comes from—according to the Latin, mala can translate as "bad", "evil", "ugly", "unlucky", "mischief", "disaster"; "apple" (hence the idea that the fruit eaten by Eve in the Bible was an apple), or even "lemon"; and sometimes "cheeks" or "jaws" which is close; but not "beard" itself, far as I could find. There are reports of a bearded Aphrodite, as an androgynous/hermaphroditic Deity associated with sexual union (and in researching that I was surprised to find out that there is a song by my beloved Andy Partridge, of XTC fame, called "Bearded Aphrodite"; I s'pose if anyone knows these weird mythological things it'd be him); and there is Mylitta of the east, also a Goddess of Sexuality, who could be shown with a beard (She is probably related to Malidthu, the Canaanite Mother of Adonis); but Fortuna Mala, is not, as far as I know, connected with sex. There is, however, a Fortuna Barbata, who was offered the first clippings of a youth's beard as a wish for luck as a boy transitioned to a man.