Answer:
I will disappear every time you say my name.
Long and thin red within with a nail at the end.
I am pronounced as one letter written with three. I come in blue black brown or grey. Reverse me and I read the same either way.
I am carried by mad animals.
my rings are not worth much but they do tell my age.
A mysterious, fantastic creature, It has one defining feature, And for humans at their worst, In their eyes it's just a horse