This applies to betta spledens (the normal bettas you get in petshops). However, there are such things as 'peaceful' betta species, such as betta imbellis and betta smaragdina. They are more costly and harder to obtain, though - chance are you won't find them in the petshops.
If your tank is big enough (ten gallons or more) and your betta is peaceful enough, there are a number of species you may be able to try. Small tetras, cories and shrimp and snails are all good starting places. Do have a backup tank if you try it, though, just in case your betta doesn't like it.
This thread is more detailed. :)