Many villains populate the story of Fullmetal Alchemist: Brotherhood, with Father the ambitious homunculus being the archvillain. He schemed for over 300 years to sacrifice all of humanity to become a new god, but he needed some minions. The firstborn was Pride.
Pride is Father's cast-off arrogance and self-love, as his name suggests, and is the strongest of Father's creations. In addition, he most closely resembles Father in his true form, appearing as a black, shadowy mass with red eyes and mouths (similar to Father's "dwarf in the flask" body). But what was Pride up to for three centuries, and how did an undying, unaging creature stay hidden?
Pride Played The Long Game
Most likely, Pride simply chose to lay low and bide his time along with Father for a long time, choosing not to draw unnecessary attention to himself. Within years of the fall of Xerxes, Father cast off his sinful pride and created the homunculus of the same name, but Father and his new "child" were in no position to suddenly wage war on humanity and ascend to godhood. Instead, Father continued to create his seven "children" over time, and Wrath was the youngest. Wrath is embodied in a weak Philosopher's Stone made up of just one human soul, able to be transplanted into living humans to gain a physical form. Unlike King Bradley/Wrath, Pride had the same form for a long time, and it is possible that he always looked the way he did in the main story. He is prideful, so why wouldn't he love his own appearance and cling to it? He must also have love the name Selim Bradley, wearing it like a badge of honor.
Father's plan required many decades of work, including the creation of his enormous transmutation circle/tunnel in the earth and the sacrificed blood of thousands of people. Over the years, many slaughters, civil wars and battles occurred at key points across the growing nation of Amestris, and Pride must have watched it all smugly as the short-lived humans slaughtered each other without even realizing that they were fueling Father's grand plan. It is unlikely that Pride grew bored or impatient during this time; instead, he probably enjoyed outliving all those "pitiful" humans, and took pride in how long he could bide his time for his father to complete his quest. Boredom, impulse and impatience are the weaknesses of mortal humans, Pride would say. A 300-year wait is no big deal.
How Pride Stayed Hidden in the Shadows
Even if Pride was patient enough to wait 300 years and watch Father's plan unfold bit by bit, he still needed to lay low. If he was revealed for what he truly was, then the people of Amestris would have tried to trace Pride back to Father, and possibly realized what Father was trying to do. Father could afford to have his plans or his minions discovered, but fortunately for him, hiding them from the world at large would have proved fairly easy. With the exception of Greed, all of the homunculi were deeply loyal to Father and had no reason to sell him out for fame, power or money. In addition, the homunculi don't need to eat or have steady employment, so they can stay hidden in remote (and possibly underground) bunkers and lairs over the years, patiently biding their time.
Pride, in particular, could not only stay hidden in his Selim Bradley form in some remote mountain lair (as well as the lair under Central), but he could also live in the giant, circular tunnel that Sloth was busy digging around the entire border of Amestris. This circular tunnel had two functions. It would later serve as a massive transmutation circle for Father's plan on the Promised Day (as the anime showed), and it doubled as Pride's bonus container. Pride could walk around as Selim Bradley but he can stretch is legs (so to speak) by living inside that massive tunnel -- free from detection by the oblivious Amestrians living above.
And as shown in the anime, Pride is quick to find and slaughter anyone who does discover the tunnel and explore it out of curiosity. Pride's massive shadow-body and his many eyes allow him to see and hear anything in a vast area, making it nearly impossible for people to discover him and escape with that knowledge. Meanwhile, Pride could (and no doubt actually did) routinely survey the humans of Amestris to note any developments and check on the progress of Father's plan. He is an entire spy network all by himself.
Eventually, as the Promised Day neared and Wrath was born as King Bradley, Pride used his Selim Bradley body to pose as Wrath's innocent son, appearing in public without a care in the world. And he was a skilled enough actor to fool anyone -- even his own "mother."