You could have run across a Crown Tail.... their tails do look a bit like spikes.. Most walmart & petsmart betta's are veiltails. There are also..Halfmoons, doubletails...lots of tail choices if you go to the right breeder to find them. Veiltails are basically your "mutt" of the breed
with proper water and lots of love these guys will heal to be beauty's again.
Hope this helped some
Fin rot looks like grey or black edging on the fins, like it's falling off in small chunks