You might not be surprised to hear that the white cross on Joey's forehead is a little bit of subtle religious imagery. Emilie's Grandpapa shines the light on it, too, as the only character to make explicit references to God and religion. He says to Joey,
"You must have prayed to that horse god of yours because my Emilie woke up. […] We've all had a Christmas present today, haven't we? All's well, I tell you. All's well." (10.14)
Even though no one else quite puts it into words, Joey is a savior for quite a few people. He's Albert's raison d'être. He protects Trooper Warren. He cares for Topthorn. The Germans practically worship him, giving him a medal of honor (which is also, not-so-coincidentally, a cross). Joey's just a little miracle on four legs, spreading love and salvation to all at times of war.