What was their mascot?

The Pittsburgh Pirates were originally called the Pittsburgh Alleghenys.

Professional baseball has been played in the Pittsburgh area since 1876. The teams of the era were "independents", barnstorming throughout the region and not affiliated with any organized league, though they did have salaries and were run as a business organization. In 1882 the strongest team in the area joined the American Association as a founding member. Their various home fields in the 19th century were in a then-separate city called Allegheny City, across the Allegheny River from Pittsburgh. The team was listed as "Allegheny" in the standings, and was sometimes called the "Alleghenys" (not the "Alleghenies") in the same generic way that teams from Boston, New York, and Chicago were sometimes called the "Bostons", the "New Yorks", and the "Chicagos", in the sportswriting style of that era. After five mediocre seasons in the A.A., Pittsburgh became the first A.A. team to switch to the older National League in 1887. At this time, the team renamed itself the Pittsburgh Alleghenys, although Allegheny remained a separate city until it was annexed by Pittsburgh in 1907. At that time, owner-manager Horace Phillips sold the team to Dennis McKnight; Phillips stayed on as manager.

In those early days, the club benefited three times from mergers with defunct clubs. The A.A. club picked up a number of players from a defunct Columbus, Ohio, team in 1885.

The Alleghenys were severely crippled during the 1890 season, when nearly all of their stars jumped to the Pittsburgh Burghers of the Players' League. With a decimated roster, the team experienced what is still the worst season in franchise history, going 23-113. The battle nearly ruined McKnight, and he was forced to return his franchise to the league. However, almost immediately after this, McKnight joined the backers of the Burghers as a minority owner, which then repurchased the Pittsburgh National League franchise and rechartered it under a different corporate name. They were thus able to legally recover the services of most of the players who had jumped to the upstart league a year earlier.

The new owners also signed several players from American Association teams. One of them was highly regarded second baseman Lou Bierbauer, who had previously played with the A.A.'s Philadelphia Athletics. The Athletics failed to include him on their reserve list, and the Alleghenys picked him up. This led to loud protests by the Athletics, and in an official complaint, an AA official claimed the Alleghenys' actions were "piratical". This incident (which is discussed at some length in The Beer and Whisky League, by David Nemec, 1994) quickly accelerated into a schism between the leagues that contributed to the demise of the A.A. Although the Alleghenys were never found guilty of wrongdoing, they made sport of being denounced for being "piratical" by renaming themselves "the Pirates" for the 1891 season. The nickname was first acknowledged on the team's uniforms in 1912.