Placebo Effect is an observation. When it is observed, we announce there is a placebo effect happening. When it isn't observe, then it is not observed.
A placebo effect should not happen reliably, and not with every person. If it does happen reliably, then the particular "Pill" used for the experiment will be validated as a new form of effective drug.
You are seeking a report validating Placebo Effect in animals.
Even if you have found a validating report, it would be limited for a particular animal, for a particular illness or disease, and with a particular drug, or therapy. Very limited as useful proof. Except that Placebo Effect do happen when it happens.
Sometime I find that people talked about placebo effect as if it a new and real drug or an established medical treatment. And that it really works. This should be avoided. It doesn't work reliably. It shouldn't work reliably.