The Petersen graph is one of the Moore graphs (regular graphs of girth 5 with the largest possible number k2 + 1 of vertices). Two other Moore graphs are known, namely the pentagon (k = 2) and the Hoffman-Singleton graph (k = 7). If there are other Moore graphs, they must have valency 57 and 3250 vertices, but cannot have a transitive group.
The Petersen graph is also a cage (graph with smallest possible number of vertices given its valency and girth).