Beta Weapons are weapons that were removed from a GTA game during its development. It also includes some weapons that didn't make it to the final game and weapons have their models changed, and some still exist in the game files. Many made well during development and it is unknown why they were removed. However, most of them can be obtained through cheat device and trainers.
In the trailer, Trevor can be seen holding an assault rifle with a laser attachment, the assault rifle made it in the final game, but the laser didn't possibly due to the fact that it is useless and is not required since the player can use the HUD instead.