I picked up my wyvern (while half submerged in water) so I can passively tame a Hesperornis, saw a raptor and went to throw my wyvern off my shoulder and noticed it wasn't there. My shoulder was empty like a dino wasn't there in the first place. I searched (and whistled) up and down the river, but he was nowhere to be found. I logged off of Ark and exited Steam, but when I came back on he was still nowhere to be found. There's nothing in the Tribe Log about his death so my theory is it's either a glitch or he blinked out of existence.