In my animal pens I would have cows and chickens seemingly dissapear. While on a lan world my brother playing with me could see on of the invisible chickens. But neither of us could see the cows. After death the cows became visible again, so did the single invisible chicken. I went to attack a spider (as it was night) and next thing I know Im being shot at by it. Apparently it was a spider jockey but the skeleton on top was invisible. my brother could see it on top the spider while I could not. After death all invisible mobs seem to reset and become visible again. But shortly after they begin to disappear again after leaving the chunks and coming back 5minutes later.