These are the mainstays of Legion armies. They're fast, deadly, numerous, and they have a suprise up their sleeves.
Legion Assault Speeders came about when 8311, seeking vehicles for the rapidly expanding Legion during its early days, came across an old abandoned Republic weapons dump. In this dump was a batch of old swamp speeders. 8311 had the weapons dump looted of anything valuable, and had the swamp speeders modified. These became the direct ancestors of the Assault Speeder.
Not long afterwards, the Legion was spreading across the galaxy, rapidly accumulating wealth and power. By now 8311 had enough money and resources to start manufucturing arms for the Legion. He built factories in several remote locations across the galaxy and began duplicating the swamp speeders, but modifying and upgrading them. 8311 meanwhile discovered plans for a cheap, low-scale cloaking device in an old CIS vault, and began applying this to the Legion Assault speeders, making them capable of becoming invisible. These made the Assault Speeders lethal vehicles.
A vast number of Assault Speeders--estimates ranged from the tens of thousands to hundreds of thousands--were built over the course of the Legion's pre-battle of Yavin history. When the Legion collapsed after the Empire's unsuccessful attempt to kill 8311, many Assault Speeders ended up on the black market. However, once the Legion returned to power, it re-aquired its old speeders.
Legion Assault Speeders were very similar the the old swamp speeder in a lot of ways, but soon Legion weapons designers brought them beyond that. They were given extra armor and more powerful weapons than their predecessor. Once cloaking devices became standard issue for the Assault Speeders, this made them even more deadly.
Assault Speeders were generally piloted by Legion soldiers or hired mercenaries. However, some were piloted by Legion Infiltrators, who made good use of its cloaking device.