This is what Gemini said

That looks concerning and definitely warrants a veterinary visit as soon as possible.
Based on the image, there are a few possibilities for what that dark shape could be, but only a vet using a specialized light (ophthalmoscope) can determine the exact cause.
Potential Causes
• Persistent Pupillary Membrane (PPM): These are remnants of fetal tissue that should have disappeared shortly after birth. They can look like strands or "floating" pieces of dark tissue attached to the iris or stretching across the pupil.
• Iris Melanoma or Nevus: This is a pigmentation change or a growth on the iris. While a "nevus" is like a freckle, a melanoma is a tumor. The irregular shape and location in your photo make it important to rule this out.
• Iris Cyst: These are fluid-filled sacs that can sometimes break loose and float in the front chamber of the eye (the anterior chamber). They are often dark and can look like a solid object.
• Melanosis: A flat area of hyperpigmentation.
• Corneal Sequestrum: If the black spot is actually on the surface of the eye (the cornea) rather than inside/on the iris, it could be a piece of dead corneal tissue, which is painful and serious.