Larger, Stronger, Yet Quite Slow
The Alpha Grim never makes an appearence in the Single Player Campaign, instead it is totally unique to the Co-Operative Game. They are much larger than the standard Grim and far more deadly. Their Cocoons as such also reflect this, and they have an unusual yellowish colouration to them.
They are essentially the Primarch command units for the Grims, although their actual stature as a Chimeran unit is unknown. Alpha Grims are generally not a problem for any team to take down. They move insanely slow and actually being hit by one requires a certain special kind of skill. Any form of weaponary will take them down just be prepared for a long slog as they have a rather large abundance of health like most other Primarchs.