They have a distinctly darker skull color and exhibit greater speed, strength and hostility.
They were coined 'Alphas' not just because they are more dangerous but because there is never more than one seen together and is usually followed around by several regular mutants, acting almost as a pack, hunting and destroying everything in their paths.
Alphas will often go into a rage and bash the hood of your car, damaging your engine and disrupting your aim temporarily.
The alpha mutan uses the same tactics as the regular mutants but does so in an even more brutal way. It's stronger, faster and tougher.
When an alpha mutant is on your hodd, they will sometimes go into a frenzy, slamming the hood, causing damage and shaking the vehicle, ruining your aim.
There is never 2 alpha mutants together but a single alpha will usually have a small group of regular following it around, like a pack.