The green horse is the equestrian statue of Marcus Aurelius. This horse is a focal point of Piazza…read moredel Campidoglio, immediately next to Basilica di Santa Maria in Aracoeli.
The statue is ancient bronze (2nd century AD). Over -2K years, bronze reacts with air and moisture, forming a green patina (copper oxidation), which
protects the metal underneath and is intentiol preserved.
In the Middle Ages, people mistakenly believed the statue depicted Emperor Constantine, the first Christian emperor.
Due to that mistake, it was not melted down, unlike many other pagan bronzes.
The statue outdoors today is a replica; the original is preserved inside the Capitoline Museums to protect it from weather damage.