Singleplayer world with other players

I mean play on single player is great i strated to play mininecraft abuot month i don, t know what is it its a mob i tried to film a video [^VID_20200512_235705.mp4]. How i know my, friend said thats hero or i dont remember.Herobrine. I finked hes are player but im scared thats singleplayer [^VID_20200512_235117.mp4][^VID_20200512_235705.mp4]aw[^VID_20200512_235705.mp4][^VID_20200512_235117.mp4][^VID_20200512_235705.mp4]