The red horse is indeed not necessary, but it is the best pet for GST.
In compare with other pets that I already tried, the red horse give you also the most exp than others.
But sometimes I was being too lazy to catch a red horse, so I catch another red mobs nearby and use them into the GST. XD

I agree that another pets have lower speed than red horse, but I have no worries, because my speed is always 0, since I have a steel hammer 600T. XD