What is the OneAgent
The OneAgent is the new agent platform within Dynatrace.
Read more specifically about the Dynatrace OneAgent.
What happened with the OneAgent for AppMon?
Since AppMon 2017 May you can choose between different agent platforms for certain agent types. See Agent Platform for more information.
Although the new agent platform shares a lot with the Dyntrace OneAgent, the OneAgent has a lot of additional benefit like auto-discovery, auto-injection, network analytics, log-anylytics, etc which is unique to Dynatrace.