* Mustangs: These are feral horses descended from horses brought to the Americas by Spanish explorers. The first horses arrived in the 1500s, so that's when the lineage of mustangs began. However, it took time for these horses to become truly feral and establish distinct herds. So, the term "mustang" in its modern sense, referring to a specific breed of wild horse, only came about over time.
* Stallions: This is a term for any intact male horse, regardless of breed or where it lives. So, stallions have been around as long as horses have existed, which is estimated to be around 55 million years ago.
Therefore:
* Mustangs have been around for around 500 years, though their specific lineage and breed characteristics developed over time.
* Stallions have existed for millions of years.