What is the strongest horse bit you can get is it a Tom Thumb?

A Tom Thumb is a fairly strong/severe bit and not one that should be used on a green horse or by a green rider. However, there are many other bits that are much stronger/severe, even dangerous to the horse. These bits range from twisted bits, to bicycle chain bits and beyond. Cathedral port curb bits are also very strong, however, in the right hands they do no harm to the horse. It really is much more about the rider than the bit.

That being said, there is no substitution for good training, no matter the severity of the bit being used.