Custom Drupal 8 Entities w/ Bundle Examples

Most Simple - The Most Simple entity with bundles possible. Easy to create (very little code), hard to use (no links).
Simple - Simple Entity with bundles that is much more usable than the Most Simple Entity. Has links, useful ListBuilders, and informative messages.
Practical - More practical entity with bundles, generated bundle permissions, custom access control, entity fields, bundle descriptions, and much better ListBuilders with useful information. Also has interfaces for the Entity classes as best practice.

Learning about Drupal 8 Events

These modules integrate with various free cat related APIs available online.