What is the title of the famous piece that begins with an iconic main theme presented by the horns?

The famous piece that begins with an iconic main theme presented by the horns is indeed Symphony No. 5 by Ludwig van Beethoven. This symphony is known for its distinctive four-note motif that is introduced right at the beginning, which is recognized widely in classical music. The use of horns to present this theme adds a powerful and dramatic quality to the opening, making it one of the most memorable and frequently referenced pieces in orchestral repertoire. The themes, rhythms, and overall structure of this symphony have profoundly influenced the development of symphonic music and remain a staple in concert programs around the world.

Symphony No. 6, Symphony No. 9, and Symphony No. 7 do not feature this specific iconic theme at the beginning, making them less suitable answers for this particular piece of music. Each of these symphonies has its own unique characteristics and themes, but none start with the prominent and recognizable horn motif associated with Symphony No. 5.