Fortuna, Roman Goddess of fate, chance, luck, and fortune, was considered a Goddess of changing character by the Romans, and as such was known under several titles describing Her whimsical or fickle nature as a Deity Who could bring good or back luck according to Her whim. I've grouped these aspects together here, then, as they are related.

Fortuna Dubia is "Dubious Fortune", "Critical Fortune", or even "Dangerous Fortune", and perhaps refers to luck or happenstance that brings mixed blessings, or which is especially unstable; She was probably propitiated to avert this kind of luck and to bring a more calm stability to things. She evidentally had a shrine in Rome on the Aventine, as there was a street there called the Vicus Fortunae Dubiae.

Fortuna Brevis, "Brief Fortune", refers to Fortune as fickle and to good luck that does not last. According to Plutarch, king Servius Tullius, a devout follower of the Goddess Fortune as he'd been born a slave but died a king, built a shrine to Fortuna Brevis, which Pultarch calls, "Little Fortune". His rationalization of Her epithet is that sometimes the slightest change, choice, or bit of luck which seems quite insignificant at the time can actually lead to great rewards or great failures. This is the luck that it easiest to make out in hindsight; and the warning is that one should not disregard these small chances but appreciate them, for they are the curious workings of the divine plan of Fate, and are more important than you think.

Fortuna Manens was named for the more constant (and more desirable) form of luck, and the title means "Enduring" or "Abiding". She was depicted on coins of the Emperor Commodus (the 2nd century Emperor called in my Classical Handbook "the worthless son of Marcus Aurelius", and who was portrayed in the movie Gladiator by Joaquin Phoenix) as an enthroned Goddess with Her usual cornucopia to symbolize abundance. With Her right hand, however, She holds a horse by its bridle. The symbolism of this is unclear; perhaps She was able to "rein in" Her capriciousness, or perhaps She is to be equated with Fortuna Equestris. There is also the outside chance that Fortuna Manens was the one you prayed to for luck in attracticing lovers, for another meaning of manens is "to stay the night to have sex"!

Fortuna Mobilis, by contrast, was the opposite of Fortuna Manens, for Her name means "Changeable", "Shifting", "Fickle", or "Easily Swayed" Fortune; She was paired with Fortuna Manens as Her balancing and opposing force.