Entity select Dropdown I'm working on a Drupal 8 view that is going to be a page. 

How can I make an exposed entity filter a dropdown? The entity is not a content type, but rather a custom entity with it's own module.

I can't seem to find any documentation about it.
Views on Drupal 8 are slightly different, but you might consider this answer (given for Exposed filters with dropdown selections), which gives you a quick module way of creating a custom node-based exposed filter drop down for a Drupal 7 view. This is essentially what I would suggest for your use case.