I tamed one of these yesterday and I, of course, fed it element after it was tamed to see what it was like. Part way through the transformation I left it by my base and went to the bog to tame a Bloodstalker. When I came back, it was gone, I used the spider vision to see if I could find it but it was gone. On the wiki it also notes that there’s a chance that tamed Ferox when transforming back into their small form, will dissappear. Because of what happened, I cheated another tamed one in (It was only level 30 so I spawned a level 20 because of the 100% effectiveness) because I spent Element on that tame which will not be wasted. Moral of the story, feeding the Ferox drugs sometimes makes it dissappear.