I've got upgrades that'll speed up the growth and provide passive growth, but moods... I suppose they'd have to be events? Events that appear at random intervals with random effects.
Maybe something like the golden cookie in cookie clicker where it appears for a short time and gives you a temporary bonus. Perhaps if you let it expire, you get a debuff instead.
...I'll consider it. I don't want the game to get too complex. At least, not at too fast a pace. Unlocking them via an upgrade later on seems perfectly viable.
For specific foxes, because the prestige hinges upon the upgrade system, I don't want to give random chances to have a much easier or harder times grinding out the next upgrade due to the specific fox's disposition.