Secret way to trigger Thesaurus on Mac OS X? Not really sure how to ask since I have no idea what just happened, but I'll try my best :-)

I just typed "I" into a textfield here on Yosemite and then tried to click on an icon in the Mac OS status bar (with the option key and some other key pressed AFAIK), but I missed and the next thing I saw was a dropdown that contained the entries "He", "She", "It", "You", "We", ...I am unable to reproduce and would like to know what this was and how to  make it happen again?
It is possible that you pressed the esc key. Yosemite has a similar feature to iOS 8's QuickType keyboard that "intelligently predicts" what you are trying to type. It looks like this:



Is this what appeared?