While I was working on one of the business requirement, I created new Custom Activity Entity and the requirement was to show on Timeline of Lead entity as well.
I though it will be visible as soon as we create entity and publish. However the result was different on UCI app. So I decided to explore and found out, In order to show Custom Activity Entity on Timeline we may have to look at below points.
Custom Activity Entity should be added to respective odel driven App. Otherwise this will not be visible on Timeline.
Open App Designer -> Select Entity-> Save and Publish
We have to select Entity in Timeline configuration of Formm Editor. Say you want this entity to be visible on Lead Timeline.
Go to Customize the system-> Entity->Lead->open Lead Form Editor-> select Social Pane->Click on properties-> Validate if new Cutom Activity Entity Selected in Activity Tab
Enable Quick Create and Enable for Unified Client check box on Entity configuration.
After configuring above steps I was able to see records on Lead Timeline.
Hope this helps!