Answer
No, MIB files do not reside on the Junos OS devices. You must download the MIB files from the Juniper Networks TechLibrary page for the required Junos OS release: Junos OS MIBs.
There are some common MIBs supported by all the Junos OS devices, such as the Interface MIB (ifTable), System MIB, and Chassis MIB. Some MIBs are supported only by functionalities on specific platforms. For example, the Bridge MIB is supported on the EX Series Ethernet Switches and the SRX Series Services Gateways for the branch.
Junos OS does not support the Entity MIB, which is designed to identify physical and logical elements of a managed device. Instead, Junos OS supports the enterprise-specific Chassis MIB to identify the chassis components on the device.
For a list of supported standard and enterprise-specific MIBs, click SNMP MIBs and Traps Reference.