Kill 1 Siam.
The first Siam you encounter will be in the Police Station parking lot, after you meet with Wheeler and trying to find Elle. You will have to kill this one to further the game.
There are only a handful of these monstrosites in the game, they are very powerful and super quick.
Their biggest weakness is the shotgun, you can fell one of these guys with 3-4 shots no matter where you hit it, as long as they are close enough (which they usually are :D)
If you don't have any shotgun ammo, use either your pistol or rifle (if you have it), they are somewhat weak to these weapons and will make them pause with every hit.
As a last result if you're out of ammo, use the Knife. Dodge its attacks and use quick counters and attacks, try and sidestep behind it to the female legs, attacking them will deal some heavy damage.