Iowa is a rival, according to this websites method of crowd-sourcing this kind of data. According to Nebraska fans, our top three rivals are Oklahoma, Wisconsin and Iowa. Likewise, Nebraska is Rival #4 for Iowa, after (obviously) Iowa State, Wisconsin and Minnesota.
What's particularly hilarious to me is the Schadenfreude column, which basically calculates how much joy we take in the misfortunes of another team. Top three are Ohio State, Iowa and Texas. Oklahoma is much further down the list in this category, essentially meaning that the Sooners are a rival, but we don't rejoice in their calamities.
Anecdotally, that all fits because I don't mind when the Sooners have success, but I absolutely revel in the idea of Iowa losing to some FCS schlub or having all sixteen of their RBs get turf toe, etc.
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Side note for the Big Ten West -
Nebraska, Iowa, Wisconsin and Minnesota ALL list each of the other 3 among their top four rivals. To me, that means we have successfully turned the Triangle of Hate into a Square. Say what you want about fit and recruiting and style of play, but within a decade we've managed to make the other 3 "good" teams in the division to see us as a rival. It's not to say that NW, Illinois and Purdue aren't worth the effort, but my gut says that in the next 50 years, probably 48 of the Big Ten West divisional titles will be split among NU/IA/MN/WI while the Land-o-Lincoln schools can share the other 2.
Start layering in some forthcoming Frost beatdowns and these rivalries will only heat up more.