Brew cask install emacs takes control of the emacs command line command
You could create an alias in zsh that invokes emacs -nw whenever you write emacs. The -nw argument means to start emacs in "non-windowed" mode - i.e. a command line text interface.

You can create the alias by editing the .zshrc file you mention, and add the following line:

alias emacs="emacs -nw"