Some endpoints allow you to create custom objects, and then let you interact with those custom object via the APIs. Salesforce and Eloqua are a few examples. These APIs are unique in that you only need the name of the object to be able to interact with it using full CRUDS (create, retrieve, update, delete, search).

For example, with these endpoints, call GET /objects. This will return a list of objects like this:

[
"Account",
"Contact",
"CustomObject__c"
]

You can then use the /{objectName} APIs to interact with the custom object. For example, you would simply call: GET /CustomObject__c.

This would retrieve a list of your custom objects just like any other search endpoint. You can apply transformations, and poll on these objects as well.