I've lost count of the amount of times someone has said to me rather excitedly that they've found a horses hoof fungus only to be presented with it and see that it's actually a ganoderma. I can understand the confusion as some artists conk really do look like Horses Hoof on the tree, especially when older and when the bottom of the fungus has hardened and lost it's soft white skin.
This is an Artists Conk (ganoderma) I've cut off the top so you can see the colour of the Amadou, this is the easiest way to tell for sure which one you've found, if it's a chocolate brown colour then it's an Artists Conk
This is a Horses Hoof (Fomes Fomentarius), you can see the difference in colour, if it's a tan or a mustard colour then it's a Horses Hoof fungus.
And side by side, it's far more noticeable like this
and you can also see the difference in the fibers in the amadou. Ganoderma on the left is coarser and fibrous while Fomes on the right is more tightly grained and finer in texture.
However both will take a spark well and both will work fine with flint and steel if you prepare the fibers correctly