Corydoras(corycatfish), zebra danios, any tetra that doesn't have long fins, rasboro. Do not place long finned fish in such as goldfish or aggressive fish such as barbs, other male bettas, gouramis or angels.
Any Fish without long fancy fins or are known "fin nippers" can go good with a Betta. . Betta's are part of the gourami family, which are known to be semi aggressive but can do fine with none aggressive fish such as Zebra Danio's, Neon Tetras or any of the platy species.
Betas are not a social fish. They are highly aggressive, and will inevitably fight with any other fish. They are best by themselves, and don't need much room.
Some people keep bettas in community tanks, and some people try mating a male and female. You cannot keep two males together even in a large tank. Apparently they are just fine on their own in a small bowl or tank, too.
Definitely not other bettas as the species is territorial, but you can try neon or cardinal tetras although not other tetras and the following: plecostomus, cory cats, mollies, and swords.
Betta Fish, or Japanese Fighting Fish, typically do best in an isolated environment. This is because their natural instinct is to acquire and protect their territory from other fish, which they view as threats. Betta Fish do not need, or want, companions.
