I get 'Command Not Found' when I try to run Android Emulator on Mac OS X

The current directory is not normally included in your $PATH on a *nix operating system like OS X; to execute a program in the current directory, precede it with the path to the current directory (.):

$ ./emulator

Emulator can be added with Android Studio https://developer.android.com/studio/run/managing-avds.html

To start emulator: ~/Library/Android/sdk/tools/emulator -avd Nexus_5X_API_23

Related question: How do I launch the Android emulator from the command line?