Using Add_image_size when adding image in post

You also need to add it to image size chooser.You can do so using the filter image_size_names_choose.

add_filter( 'image_size_names_choose', 'wpse_228675_custom_size' );

function wpse_228675_custom_size( $sizes ) {
    return array_merge( $sizes, array(
        'image_horizontal' => __('Custom image size'),
    ) );
}