You can find Cajun Shrimp on the menu at
Domino's as a regional offering in Louisiana around Lent. One of your fellow BrandEaters was kind enough to snap a photo of a flyer promoting the shrimp offer.
Basically you can add Cajun shrimp to any medium, large, or extra-large pizza for an extra $2. There's also a large Cajun Shrimp Feast Pizza for $11.99.
Cajun Shrimp is offered at participating Domino's locations through April 5, 2015.
As far as I can tell, they offer this every year. The earliest mention I can find of it online is from 2013 though.