Hello Carole,
Really do need photos to tell for sure what it is but it could be excess sole. Excess sole is not necessarily a bad thing. It can be a good temporary protection  while good live sole grows underneath. Usually the excess sole will dry and separate away from the good live sole underneath and you can lift it off with a hoof knife leaving a nice natural concave sole. I only trim my own horses so I leave the excess sole alone until I can lift it all the way to the frog and it breaks free there and at the bars. Then I use my 4" grinder to smooth down the bars, lower the heels, maybe scoop the quarters a bit then backup the toes and finish with a mustang roll. I never touch that new sole.
Don Huston

At 08:47 AM 10/7/2007 Sunday, you wrote:

One of my horses has this stuff built up on the bottom outside edge of
his feet, it is almost the width of a shoe. What is it, why is there
and what caused it, and is it bad? His hoof prints look like he has
shoes on. He has nice tuff feet, two farriers who trimmed him in the
past commented on his good feet.never takes a lame step.
Just curious
Carol