So it looks like you are using the Entity Reference field. The target types, are Drupal Entity types. You can definitely create your own custom entities.
The Drupal Examples module has an entity example: https://api.drupal.org/api/examples/entity_example%21entity_example.module/group/entity_example/7.x-1.x
Although this doesn't leverage the additional tooling provided by the Entity API module...
If you are wanting to reference CiviCRM data from Drupal node, install the CiviCRM Entity module. CiviCRM Entity exposes CiviCRM API entities as Drupal entity types
With that enabled you can reference CiviCRM memberships, contacts etc...Or create new ones, using Inline Entity Form widget