something weird happened

I had recently spawned a new world in single-player Minecraft. Everything was normal at first as I was chopping down trees and making a crafting table. I noticed something among the dense fog (I have a very slow phone so I have to play with a  tiny render distance). I thought it was a cow, so I pursued it, hoping to grab some hides for amour. It wasn't a cow though. Looking back at me was another character with the default skin, but his eyes were empty. I saw no name pop up, and I double-checked to make sure I wasn't in multiplayer mode. He didn't stay long, he looked at me and quickly ran off into the fog. I pursued out of curiosity, but he was gone. I continued on with the game, not sure what to think. As I expanded the world I saw things that seemed out of place for the random map generator to make; 2x2  tunnels in the rocks, small perfect pyramids made of sand in the ocean, and groves of trees with all their leaves cut off. I would constantly think I saw the other "player"  in the deep fog, but I never got a better look at him. I tried increasing my render distance to far whenever I thought I saw him, but no avail. Hears a  screenshot below. He's at the left of the hill past the fog. Tell me who he is by tomorrow. Bye.