If you upgrade IBM Informix® to 12.10 version, you will see that some binaries that exist in previous version are now missing.
Not only onmonitor but the ISM have been removed from the 12.10 product. IBM decided to remove because it does not support most of the new onconfig parameters and many onparams / onspaces features.
Resolving the problem
IBM Informix Primary Storage Manager (PSM) replaced IBM Informix Storage Manager (ISM). It is easier to set up and use, even in embedded environments.
Open Admin Tool has all and more utilities as onmonitor.