When the Macbook Pro connects to an external LCD monitor, fonts look very strange in some program (Like Terminal) on the external monitor. It happened on 2015 Macbook Pro w/ OS X El Capitan 10.11.6. Monitor AOC E2429SWHE.
It appears the fonts on an external monitor are not smoothing because system mistakenly recognizes the monitor as CRT.
To solve: In Terminal
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
2 here corresponds to Medium – Best for Flat Panel. You may also use 1 for light smoothing, and 3 for strong smoothing, as per the original OS X font smoothing options.
Relaunch the program to take effect.