Rhinos love to wallow as much as they love to scratch their horn, especially the base of the horn. Every individual has their favoured “rubbing rock” or pumice stone, just like they have a preferred wallowing mud hole or rubbing post. White rhinos are often observed at a watering hole and the drill seems to be: drink first, then wallow and rub horn in mud, then scratch horn on rock and rub belly and in between legs on a tree stump.
Periodically we find a rock in the middle of a road, and we stop and move it so that we can drive past, only to drive the same road the next day and find the rock in the middle of the road again! The rhinos move it right back on the road as they push it around rubbing the base of the horn.
Sanding the horn like this is important because they rub the outer softer part which exposes the hard inner core, and that is sharpened into a point. The sharpened horns are used by males in territorial or discipline disputes. It is quite something to see rhinos fighting as there is much rumbling and vocalisations, bursts of speed, clouds of dust and sometimes the most terrible wounds which can lead to death if vital organs are punctured or infection sets in.
I have observed that often old males seem to have shorter horns than old females, because it seems like the males rub and shape their horns much more frequently.