According to the The Sims Wiki there is a chance the Zenyatta horse gnome will spawn whenever a horse licks a saltlick for 80 minutes. The chance increases by 0.2% for every 20 Sim minutes the horse licks the salt lick.
When my horses obtain sufficient Lifetime Rewards points I invest in the Celestial Saltlick, which satisfies their hunger and thirst and can give them a positive moodlet. Since I don't use the self-cleaning box stalls, this is a nice alternative.
While I've yet to verify the difference precisely, I can report that the Celestial Saltlick spawns horse gnomes at a rate greater than that given by the Wiki for normal saltlicks. At present, I have four Celestial Saltlicks and each one spawns a gnome each Sim day -- that's over 4,000 extra simoleans every day when they are sold. It goes without saying that these little gnomes have played a huge role in helping me to build my equine empire.
Value of the gnomes is about 1,014 simoleans at spawn time. The value does degrade over time, so don't let them sit around.
