Reason Betta Pectoral Fin Could Disapear?

[learis]

I just realized my female betta's left pectoral fin is gone. She's in a 10 gallon with 5 ember tetra. I've never seen them fight, and I can't imagine that one of those tiny fish could attack her to the point of taking her entire pectoral fin.

I was wondering if anyone knew any other reasons it could go missing, and if it will grow back. From the outside she looks completely normal.

[madagascariensis]

the fin could have eroded away from water quality issues, or if
the betta is new and you have never given any time to previously observe her closely it may be that she simply never ever had a fin. inbreeding due to mass-producing of fish leads to genetic deformities.

if the base of the fin still is there then the fin has the potential to grow back.