'Old Harrovians' is the common term for former pupils of the famous Harrow School in the UK. The term Harrovian is also used on its own, but when referring to alumni of Harrow, Old Harrovian is preferred; this is common practice, and you likewise speak of 'Old Etonians' and 'Old Reptonians'.
The Harrow School is one of the oldest and most respected of boarding schools for boys in England. It is, unsurprisingly, located in Harrow. The School was founded in 1572 by John Lyon, and has been an effective old boy network across centuries and tying together generations of rich toffs.