Etymology of "Agent" and "Proxy": RFC1095

The  terms, agent and proxy, received their fullest treatment in 1989 in RFC1095 (The Common Management Information Services and Protocol over TCP/IP)

There reside application processes called "agents" on managed systems.... Network management occurs when managers and agents conspire (via protocols and a shared conceptual schema) to exchange monitoring and control information useful to the management of a network and its components. [RFC1095]

Proxy is explicitly placed in a legal context.

Proxy is a term that originated in the legal community to indicate an entity empowered to perform actions on behalf of another. In our context, a proxy is a manager empowered to perform actions on behalf of another manager. ... the proxy assumes the agent role with respect to the requesting manager and the manager role with respect to the managed device.  [RFC1095]