A girl who, despite having 'the goods', just isn't attractive. Typically as a result of personality/intelligence or a complete lack thereof.
It is perfectly normal to find a northern pike 'hot' but the thought of dating one ought to induce near suicidal nausea
Derived from the song "She ain't pretty (she just looks that way)" by the Northern Pikes
Bob: "So how was your date with that hottie from the bar?"
Steve: "Total waste of time. She was such a northern pike that had to duck out of dinner before she made me I stabbed my eyes out with a fork."