Move to the side and allow them to pass. Some horses are easily frightened by bicycles. A frightened horse can be a danger to its rider and to you. Please take care to avoid conflicts and accidents. Do not ride in large groups.
Small groups are more manageable.
Skidding can lead to erosion by removing the surface layer. Keep your bike clean. This will prevent the spread of weeds and plan diseases and help maintain your bike. Avoid riding in wet, muddy conditions. The tracks you leave behind channel rain water and lead to erosion.