Variable: fringe-cursor-alist

Mapping from logical to physical fringe cursor bitmaps.
The value is an alist where each element (CURSOR . BITMAP)
specifies the fringe bitmaps used to display a specific logical
cursor type in the fringe.

CURSOR specifies the logical cursor type which is one of the following
symbols: `box' , `hollow', `bar', `hbar', or `hollow-small'. The last
one is used to show a hollow cursor on narrow lines display lines
where the normal hollow cursor will not fit.

BITMAP is the corresponding fringe bitmap shown for the logical cursor type.