Call this the Trojan Horse On Ice ... a bunch of protesters snuck "Chick-fil-A Is Anti-Gay" signs onto the fast food restaurant's Zamboni at a pro hockey game Saturday -- and the video is hilarious!!!

It all went down at this weekend's Cincinnati Cyclones game ... where fans usually pay a small fee to ride in the Zamboni during intermission with fun signs.

The problem for the Cyclones? They say fans concealed anti-Chick-fil-A messages inside of "Happy Birthday" signs ... and then broke out the protesting ones once they were on the ice.