What herds posed the greatest threat to cowhands on cattle drives?

The cattle herd they were herding. The reason for this was because all of these cattle were feral animals, always prone to panicking if something unfamiliar, be it a sight, sound or smell, crosses their path or gets in their line of vision. When one animal starts panicking, it causes a chain reaction causing all the animals to panic and get into an uncontrollable stampede.

However, large buffalo (or rather, American bison) herds were also huge threats to cowhands on cattle drives because they were not only easy to stampede--a little easier than the feral cattle they were herding to the stockyards--but also they would charge at a fraction of a second's notice and attempt to kill both horse and rider. Such dangerous herds would also drive off the cattle themselves, sending them into many different herds in many different directions, which would take days or even weeks for cowboys to round up and get back together again.