Just noticed this in my geldings hoof. Not too sure what it is. Didnt smell any foul odors and he doesn't seem lame. I can stick most of the hoof pick inside. He had shoes which were pulled. Farrier didnt say anything bout it last week but he was also being a brat for the farrier too.
Google deep sulcus thrush. Even if there isnt an infection in there now, best to treat it and allow it to open up since its a perfect closed environment for anaerobic nasties to thrive.
I like to scrub the foot and frog well with some good soapy water in a spray bottle (plain blue dawn works great. Soak it, scrub it out well with your hoof pick brush and then rinse with the soapy water also till it is clean and runs clear. No need to rinse the soap away tho. Remember what your mom told you about washing your hands to keep germs away!) and then stuff a cottonball in the hole soaked in thrushbuster if the horse isnt sore in there till the cottonball no longer will stay in.
Also, this horses heels appear to be quite high. The trim is just as important as treatment so the frog can function well and self clean. A healthy foot will pop out the dirt as the horse moves around. High heels allow for crud to stay in there and the frog not to function properly and get stimulation.