I just had one of those DUH moments. Since my human sim is also a gardener/nectar maker I routinely keep a flame fruit in her inventory for the 10-point moodlet boost. So, while she was brushing her horse I tried dragging the flame fruit from her inventory onto the horse and - voila! - the horse accepted the flame fruit into his inventory and got a nice little 5-point moodlet boost, which is permanent so long as the fruit remains in his inventory.
I haven't tried this on cats or dogs yet, but I've no reason to believe it won't work with them as well.
(I apologize if this has been posted before -- I did search for it, but couldn't find anything.)
