Complex sentence: They agreed to let him remain a stallion, and he sired several cream-colored foals, though only one was registered: a colt named Yancy No.3, whose dam was a black mare of Percheron breeding.
Split into simpler sentences: They agreed to let him remain a stallion, and he sired several cream-colored foals, though only one was registered. This was a colt, named Yancy No.3, and his dam was a black mare of Percheron breeding.