-The Red Balloon On a Manticore training run, the X-5's found a red balloon with the words "Happy Birthday" on it. The balloon was taken away by a soldier who called it "contraband." Years later, Max saw an image of a similar red balloon in a photo of Alina and Nathan Herrero. Max decided to confront Alina on her escape (she got a briefcase full of money from killing her own father, and Max caused the money to fly out, leaving Alina with only enough to leave the city), driven by the image and promise of the red balloon ("C.R.E.A.M.").
-The Crow On another Manticore trianing run, the X-5's saw a crow and were mesmerized by it's caw. One of the kids shot at it and missed, accidentally killing another X-5. Years later, another crow appeared just before a group of X-5's were leaving to take down Manticore. Zack saw the crow and shot at it, missing again. The X-5's knew that death followed the crow's apearance, and Max's death almost followed. Zack sacrificed himself to save her ("And Jesus Brought A Casserole").
-The Manticore Symbol The symbol is seen throughout the series (In caves dating back to 50,000 B.C., on Sandman's cane, on Joshua's body). It is an image of a Chimera, a mythological creature with the attributes of several different animals. the genetically-engineered children at Manticore were also called Chimeras since they had traces of DNA from different animals. These include feline, canine, amphibian and other DNA. Apparently, since the symbols date back thousands of years, some sort efforts of producing a superior race have been goin on for a long time.