Another joint exercise, known as Foal Eagle, has been under way since the beginning of March.
In 1997, a creature said to resemble a "cat-like beast" savaged a foal in Felingwm, Carmarthen.
The ongoing annual drills, called Foal Eagle, are to finish at the end of April.
The two countries also are running separate Foal Eagle drills in South Korea through April 30.
When a horse gives birth, its foal gets up and starts walking within 30 minutes or so.
We've seen mares in the middle of giving birth and both the foal and the mare have died.
They will be followed later in the week by another set of joint military exercises called Foal Eagle.
BBC: North Korea warning as US and S Korea begin joint drill
Immediately following the birth, mare and foal were reported to be doing fine.
Sticky GI(ph) is a foal by Lost Soldier and out of Super Glued.
Recent incidents include an attack that killed a mare and its unborn foal.
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He heads to a farm, where he arrives in time to assist with the birth of a wobbly Clydesdale foal.
So it should have been no surprise that it was Zenyatta herself who broke the news of her first foal.
Pyongyang has been angered by tightened UN sanctions imposed after its 12 February nuclear test, as well as the "Foal Eagle" drills.
BBC: US and South Korea wrap up "Foal Eagle" military drills
Every individual horse adds up: fees to register a foal, and fees to enter the individual in AQHA-sanctioned shows thru its lifetime, etc.
Moves such as keeping nuclear-capable military assets in the region could unnecessarily prolong the risk of miscalculation after the Foal Eagle exercises end.
Thurman also oversees the annual military maneuvers under way, known as Foal Eagle, as well as an accompanying war game, called Key Resolve.
The Pentagon struggled over the decision to deploy the bombers as part of the war games, called Foal Eagle, according to U.S. officials.
It was announced last week that she would be bred for the first time to Tapit, to whom Zenyatta is currently in foal.
He expects to see the New York foal crop increase over the next several years, bucking the trend of a lowered foal crop nationwide.
Rachel Alexandra foaled a filly early Tuesday morning, reported by the farm to have weighed 140 pounds, the biggest foal of the year so far at Stonestreet.
Though the maneuvers, called Foal Eagle, are held regularly, North Korea was particularly angry over their inclusion this year of nuclear-capable B-2 stealth bombers and F-22 fighters.
He's already mated with Zenyatta, the 2010 Horse of the Year, whose foal, due in late March, may steal some magazine pages from Kate and William's baby.
WSJ: Tapit: Horse Racing's Top Bachelor��2013 Kentucky Derby
The scripted actions included an increased show of U.S. military force -- such as the flying of B-2 bombers -- during the annual U.S.-South Korea military exercise, the Foal Eagle.
During ongoing Foal Eagle military maneuvers in South Korea, two U.S. B-2 strategic stealth bombers, flying from their base in Missouri, conducted a mock bombing run on a South Korean range.
Pope chose Tapit to sire the filly's first foal.
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Last year, following the birth of her first foal, a colt, Rachel Alexandra also spent time at Rood and Riddle for what was described as a precautionary measure for pain management after her delivery.
"The Society holds a DNA sample for every registered foal born since that date should it be required to identify a pony if for any reason the microchip becomes unreadable, " said its secretary Susan McGeever.
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"Eyebrows started to go up when it was clear Foal Eagle was going to be protected from the budget cuts of sequestration, " the official said, referring to the forced federal spending cuts that went into effect in March.
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