An example of a Shortcodable DataObject. See https://github.com/sheadawson/silverstripe-shortcodable

config.yml

Shortcodable:
  # essential - classes that your want to be shortcodable
  shortcodable_classes:
    - ImageGallery
  # optional - additional htmleditorfields
  htmleditor_names:
    - cwp

ImageGallery.php

<?php
/**
 * ImageGallery
 *
 * An example of a Shortcodable DataObject.
 * See https://github.com/sheadawson/silverstripe-shortcodable
 **/
class ImageGallery extends DataObject
{
    private static $db = array(
        'Title' => 'Varchar'
    );

    /**
     * Parse the shortcode and render as a string, probably with a template
     * @param array $attributes the list of attributes of the shortcode
     * @param string $content the shortcode content
     * @param ShortcodeParser $parser the ShortcodeParser instance
     * @param string $shortcode the raw shortcode being parsed
     * @return String
     **/
    public static function parse_shortcode($attributes, $content, $parser, $shortcode)
    {
        // check the gallery exists
        if (isset($attributes['id']) && $gallery = ImageGallery::get()->byID($attributes['id'])) {
            // collect custom attributes from the shortcode popup form
            $data = array();
            if (isset($attributes['Style'])) {
                $data['Style'] = $attributes['Style'];
            }

            // render with template
            return $gallery->customise($data)->renderWith('ImageGallery');
        }
    }

    /**
     * Returns a list of fields for editing the shortcode's attributes
     * in the insert shortcode popup window
     * 
     * @return Fieldlist
     **/
    public function getShortcodeFields()
    {
        return FieldList::create(
            DropdownField::create(
                'Style',
                'Gallery Style',
                array('Carousel' => 'Carousel', 'Lightbox' => 'Lightbox')
            )
        );
    }

    /**
     * Returns a link to an image to be displayed as a placeholder in the editor
     * In this example we make easy work of this task by using the placehold.it service
     * But you could also return a link to an image in the filesystem - perharps the first
     * image in this ImageGallery
     * a placeholder
     * 
     * @param array $attributes the list of attributes of the shortcode
     * @return String
     **/
    public function getShortcodePlaceHolder($attributes)
    {
        $text = $this->Title;
        if (isset($attributes['Style'])) {
            $text .= ' (' . $attributes['Style'] . ')';
        }

        $params = array(
            'txt' => $text,
            'w' => '400',
            'h' => '200',
            'txtsize' => '27',
            'bg' => '000000',
            'txtclr' => 'cccccc'
        );

        return 'https://placeholdit.imgix.net/~text?' . http_build_query($params);
    }
    
    /**
     * If you would like to customise or filter the list of available shortcodable 
     * DataObject records available in the dropdown, you can supply a custom 
     * getShortcodableRecords method on your shortcodable DataObject. The method should 
     * return an associative array suitable for the DropdownField.
     * 
     * @return array
     */
    public function getShortcodableRecords() {
	    return ImageGallery::get()->filter('SomeField', 'SomeValue')->map()->toArray();
    }
}

https://placeholdit.imgix.net does not work any longer. Developer should find alternative.