function horizontal_category_func( $parent, $content = null ) {

    extract(shortcode_atts(array(
        "parent" => 'ID'
    ), $parent));

    $slug = $parent->slug;
    $cat_name = get_the_category_by_ID( $parent );
    $cat_title = "<h3><a href=\"category/$slug\">$cat_name</a></h3>";
    $cat_list = wp_list_categories( "child_of=$parent&show_count=0&title_li=&echo=0" );

    return $cat_list;
}

add_shortcode( 'horizontal_category', 'horizontal_category_func' );