How did Takeo Spikes build such a large neck?

Takeo himself said on the ESPN Football Today podcast (dated 1/18/2012 at 16:45 min mark), that he started working out his neck in High School after one of his coaches told him you need a strong core and neck to hit harder and avoid getting neck stingers and shoulder problems.

He said he would use a 45 lb plate and lay off the end of a bench and do his neck exercises like that. Basically, it sounds like he would be lying on a bench Face UP with a 45 lb Plate on his forehead and do Neck Resistance workouts. He probably also did this Lying Face Down as well, just with lighter weight as 45 lbs is a lot of weight to support with your neck.   If you're just starting out, I'd go with 5 or 10 lbs and try to bang out 15 to 20 reps (you'll feel it). Also, I would use a towel or padding for the weight to rest on, because resting up to 20 lbs or more directly on your head will hurt.

So that's how Takeo got his crazy side neck muscles.

A big strong neck will also help tremedously in avoiding concussions. 

See neck harnesses and the best neck strengthening tool the Iron Neck.