I keep having this issue! I've had two golden chickens who are well fed and happy disappear after I asked them to help. Did not take them to a fair and they are not in my inventory. Also, they always disappear when another golden chick grows into a chicken. Definitely is a bug.
I noticed that golden chickens if asked to help in the garden, after evolving all plants to "Perfect" in the garden wander away and take care of plants in the neighborhood. I focused the camera on it and actually saw it next to a tulip plant doing its magic. They do not respond to the command to go to the coop or "call over" because they are too far or because the order to take care of the garden overrides all other orders, not sure which. Sim has to walk all the way to the chicken and deliver the order "go to the coop" in person. :-) If it is not done promptly, I think, the chicken will disappear. So, I had to surround the property with a fence and lock the gates for chickens. This is definitely a glitch and should be fixed.
