While splendens are by far the most popular species of betta, there are hundreds of other types. Bettas are divided into two groups, Bubble nesters and mouthbrooders.
Some of the common bubblenesters include betta imbellis and betta smaradgina. Both of these types are wild bettas that are naturally short finned. They are becomming more popular due to their bright coloration and tint. Betta imbellis can be breed with splendens to create what is know as a hybred. These bettas are becomming very popular due to the long fins that the splendens give, and the color and shine that imbellis brings.
The way to tell imbellis apart from a wild splenden is by looking at the gill covers. Red coloration on the ear flap is red when excited. If it is shine blue, than it is a wild imbellis. Imbellis also have a sharper dorsal fin than the rounded betta splendens.
Betta smaradgina are one of my favorites. They are somewhat more peaceful, you can house 2 males with serveral females. They have great colors and longer finnage than imbellis. Cross breeding smaradgina and splenens is to my knowledge not popular, or can even be done.
Mouthbrooders is a group of bettas that instead of making bubblenest, carry the eggs/babies in their mouth. These bettas are far less popular, but ever growing. These bettas tend not to be as shinny or colorful. Betta macrostoma is probably the most common mouthbrooder. These are often expensive bettas from boreno. The breeding habits of the macrostoma is quite wierd. The pair will often do a dance and shift who holds the eggs.
These are just 4 of the many types of wild bettas out there. If you would like to learn more about these types of bettas, it would be good to check out aquabid.com and look at the wild bettas section.