I have no idea what this one is, but it sure is spectacular looking inside of it! :)

Looks like a Louisiana or Species Iris, but not sure.

It looks like a distorted historic bearded iris bloom. It could have been affected by many different factors.

Roundup, late frosts, and just lack of nutrients can cause the flowers to either be deformed/misshapen or even lack the full sets of 3 (falls, standards, style arms, ...).

A less likely possibility is that this could have grown from seed that was pollinated by insects and fell amongst your iris and grew to blooming size. You could believe that with more certainty if you separated the unusual plant and it performed consistently in this way.

It's most likely one of the other factors though and shouldn't happen reliably. Still neat to look at.