A lot of people believe that this was the trombone but as the following extract shows it could also have been the piccolo or in reality neither!
While it is commonly stated that the last movement of Beethoven's Fifth is the first time the trombone and the piccolo were used in a concert symphony, it is not true. The Swedish composer Joachim Nicholas Eggert specified trombones for his Symphony in E-flat major written in 1807[23], and examples of earlier symphonies with a part for piccolo abound, including Michael Haydn's Symphony no. 19 in C major composed in August 1773.