Growlers are jugs of beer from local brewpubs. They're the prizes for the thing, not the thing itself.
Ninja primes started as a joke because the Cat 6 guys were going off the front on non-Prime laps just for looks (originally called "photo-primes").
When they started putting on the State Fair crit they decided to make them official. Ninja primes are unannounced Primes - you don't know you've won until you've crossed the line (and the guy dressed in the ninja costume starts wailing on his electric guitar). As far as I know, there are no regular primes in that race.
Take a flyer off the front. Your impatience may be rewarded...