After seeing Momo's dedication to finding Appa and the creators saying (after being asked about Zutara) they should really wonder about Mappa.
So.... the only problem is:
A. I'm not sure if one's a girl.
B. Momo is about a 500th of Appa's size.
Remember when they got separeted in the tunnels???
They came back and talked with eachother.
I think they are just good friends but if Momo or Appa turn out to be female then hey, Mappa's possible.