UPDATE: This is a glitch that will be resolved when FarmVille "enhances" with tonight's Updates. You will not have to do anything to restore the White Stallions functionality.
From FarmVille's Community Manager, Grimwell:
"I'd like to thank everyone who has sent me their UID so far, and ask that nobody else does!
The development team has found the bug that is causing this issue with your stallions, and intend to have a fix for it later today. You will not have to do anything to restore functionality to your stallions, the update later today will care for it. Thank you for your patience, and special thanks to everyone who shared their ID's so we could get a look!" (Source: Official FarmVille Forum)
Lots of FarmVille Freaks, such as FarmVille Freak Lisa, FarmVille Freak Julia, FarmVille Freak Pia, and FarmVille Freak Becky, among others are reporting that their White Stallions are no longer producing Foals!
As you know, the White Stallion was a special limited edition farm cash animal available only through a Mystery Box. Unlike other Wandering Stallions, the White Stallion is a permanent Horse Stable occupant and once purchased you are able to keep it forever. Owners of White Stallions have the advantage of being able to produce a Foal everyday. The ability to produce foals explains why the temporary "neutering" of White Stallions, has caused angry FarmVille Horse breeder enthusiasts and White Stallion owners.
We are waiting on an official response from Zynga and will keep you informed and updated as we learn more information about the questionable fertility of the White Stallion.