Can I found out what entity types are available by calling Dynamics 365 API?
Practically I want this list. https://msdn.microsoft.com/da-dk/library/mt593046.aspx#bkmk_DerivedEntityTypes
You can get entity list using Web API. Here is description of it. For example you will get all LogicalNames from this:
http://[SERVER_NAME]/[ORG_NAME]/api/data/v8.1/EntityDefinitions?$select=LogicalName