Oh Jan -- that's funny --
Sorry for laughing -- you reminded me of my early Iris days when I wondered the same thing!!
That is a seed pod. If an Iris is happy, like most plants it will try to reproduce. This is what Iris hybridizers are trying to get -- and likely what a busy bee produced for you. If you're interested in propagation, you let that pod dry on the stalk. When it is ready, it will split and there may be Iris seeds inside. Properly cared for, they will produce little baby Iris plants that will bloom in two or three years. It's a bit of a job to get the seeds to sprout -- but could be well worth it. And we have people here on the list who can tell you how to get best results in that area.
Congratulations on your seed pod!!!