Unfortunately there are no D365 online app offerings from Microsoft with no end date.
Following are available options:
1. Standrd trial: valid for 30 days and can be extended for 30 more days (on request)
2. Partner benefit offerings : Gold partner of Microsoft receives some benefits, one of them is D365 instance. This will be valid for one year. This can be extended based on your partnership validity
If you want to get a trial for on-premise, you can download the Dynamic 365 Server and install (you should have all other pre-requisites installed like windows server, SQL, etc.). On-premise server trial is valid for 90days.
