Fumito Ueda's games are all linked in some way, and with the new Shadow of the Colossus on PS4 there's now a retroactive Last Guardian Easter egg.
You can see how to find it in the video above, but the main thing is to head to this point on the map, to the right of the main area outside the temple.
Follow the instructions in the video and you'll find a forested area. Head in and you'll see a shrine to the left shortly after you enter, and near to that a broken tree with a trail running by it. Follow the trail and you'll be led to a cave in the ground which contains a glowing, leaky barrel at the back that looks a lot like the ones Trico loves to munch so much.
Finding it will also net you a trophy called 'Boon of the Nomad'