AppImages are only for Linux based systems:
https://appimage.org/
The AppImageKit AppImage can be downloaded from:
https://github.com/AppImage/AppImageKit/releases
It is recommended to build AppImages on older distributions:
https://docs.appimage.org/introduction/concepts.html#build-on-old-systems-run-on-newer-systems
At the time of this document, CentOS 6 is the one of the oldest supported Linux distributions with the oldest libc version.
Use a custom configured PostgreSQL build with minimal options enabled to reduce library dependency support. Part of this reason is to make it easier to include libraries with compatible licences. Part of the reason is to be able to build libpq with minimal development dependencies for PostgreSQL, since the purpose is just to build a pg_top AppImage. At least version PostgreSQL 10 should be used so that PGHOST can be defined in the AppRun script to search multiple potential unixsock directories if no PGHOST is defined in the user's environment.
At the time of this document, PostgreSQL 10 was configured with the following options:
./configure --without-ldap --without-readline --without-zlib \
--without-gssapi --with-openssl
Don't forget you may have to set both PATH and LD_LIBRARY_PATH appropriately depending on where the custom build of PostgreSQL is installed.