LEXINGTON, Ky. — At 2:08 p.m. last Wednesday, the heavy wooden door to one of the two breeding sheds at Spendthrift Farm rolls open. In walks a bay-colored, boxy horse with a prominent white blaze on his face and white markings on two of his feet. Into Mischief has arrived, ready to live up to his name and promiscuous reputation. The shed is modern and spacious. The floor is covered in a deep layer of dark wood chips, springy underfoot (and hoof). The walls are padded and the ceiling is vaulted. As equine bachelor pads go, this is swanky. “It’s a little grand for horse sex,” quips Spendthrift CEO and general manager Ned Toffey. “But the boss [late Spendthrift owner B. Wayne Hughes] wanted it to have some ‘wow’ factor.” Into Mischief was part of the celebrations of Authentic’s 2020 Kentucky Derby win, the first time one of his offspring won a Triple Crown race.…