I adore Petronella; there are a few in my family tree! [name_m]Petey[/name_m] is so genius!! 
A few ideas from my list…
[name_f]Etta[/name_f]
[name_f]Ettie[/name_f]
[name_f]Nella[/name_f]
[name_f]Nell[/name_f] or [name_f]Nelle[/name_f]
[name_f]Peach[/name_f]
[name_f]Peaches[/name_f]
[name_f]Pear[/name_f]
[name_f]Pella[/name_f] (a Scandinavian pet form, I believe!)
[name_f]Pepper[/name_f]
[name_m]Pet[/name_m]
[name_m]Pip[/name_m]
[name_f]Pippi[/name_f]
[name_m]Pippin[/name_m]
[name_f]Plum[/name_f]
[name_f]Ronja[/name_f]
Have a great day! 