Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Backdoor.ForShare is a detection for a Trojan with capable of opening a backdoor on compromised machines and downloading additional potentially malicious files.
Backdoor.ForShare is also capable of deleting user accounts and files, and monitoring user keystrokes and screens.
Backdoor.ForShare is no longer found in the wild.
Malwarebytes can detect and remove Backdoor.ForShare without further user interaction.