If you're describing what I think you are....... then it would be a cutaneous horn.

Jack and Bear both have these, I have trimmed them off before, although not flush with the skin. It did not bleed or bother either cat to have them trimmed and they seem just as fine if I leave them alone also. They were callous-y feeling/looking growths. The ones Jack & Bear have are on the actual toe pad of the foot under where nails grow, sometimes growing upwards to meet the claw. A claw grows from above the toe pad.

As far as being something that goes with FIV/FELV... Bear (has always been a house cat and vaccinated young) and Jack (has already tested negative for FELV and FIV then vaccinated).

pix of horned paws

Q&A on horned paws

Hope those links help especially the one with pix.