This is a Left Front foot with the inside wall curving to the inside, in a bell-shaped fashion (red dotted line Fig. 2). This is being caused in part by the sole at the inside toe which is too high (Fig. 1) as well as the toe too long.
This young mustang, never shod, has very thick and extremely hard walls which do not break off with wear but continue growing in a distorted way, and altering the horse’s stance.
Fig. 1 Fig. 2
The trimming treats this condition similarly to a flare but also lowers the toe on the inside (sole is removed). So, there are cases when sole forward of the frog does need to be rasped. The trimming is complicated by the inside-growing direction the walls have adopted. Now that the inside wall is growing more correctly, the outside wall needs to begin to start growing in a more outwardly direction (more visible in Fig 3 due to the angle of the photograph). Advanced trimming techniques such as floating the heel and the diagonal toe will allow this to occur.
Fig. 3 Fig. 4