What happened to my betta's tail?

It doesn't look like fin rot..my beta has fin rot because I got him from a fish store where he was kept in a tiny plastic container and theres black spots on his tail and fin rot takes a longer time to affect the fish. Your beta most likely tore its fin on something in the tank.

I can tell by that picture your keeping him in a Aqueon 1/2 gallon bowl.

That is not suitable for a betta, they need a 2.5 gallon or larger tank with a heater and filter. Please do your research before buying fish. That poor thing is suffering.

Also you probabaly have some unnecessary plastic plant that has ripped his fins.

Get a 5 gallon starter kit at petsmart ASAP!!

I suggest you remove any plastic plants. Though fist check that the tank has clean water, zero ammonia, zero nitrite, nitrates not over 20. Tempurature stable at a temp between 78 and 82.

you need a bigger tank and silk or live plants. You obviously have internet access so why not do 5 minutes of BETTA FISH CARE.

It is just ripped. In the future it might turn into fin rot and if that happens go to any petstore and buy some bettafix.

It looks ripped. Try to prevent it from rotting.