Why does Saint Peter have a rosary for his symbol?

The rosary is not a symbol of Saint Peter. These are his symbols:
book
cock or rooster
reversed cross
keys of Heaven
keys
pallium
papal vestments
Apostle holding a book
Apostle holding a scroll
cornerstone
bald man, often with a fringe of hair on the sides and a tuft on top
man crucified head downwards
man holding a key or keys
pope and bearing keys and a double-barred cross