Hello, @Bhunesh r.
AppMon Agent specifically designed for a particular technology and/or application layer whereas a single OneAgent per host is required to collect all relevant monitoring data of supported technologies.
There are also many other advantages Dynatrace OneAgent has. Have a look at the below links for more insight.
I like this link as a summary of what you get with Dynatrace. A few things overlap with AppMon but the majority is unique to Dynatrace.