City went ahead early on but then conceded two first-half goals and had defender Paul Green red-carded.
But Horsfield, playing his first game since recovering from testicular cancer, battled away and set up City's second-half equaliser.
Lincoln started well and went ahead after only six minutes. Scott Kerr was allowed to run at the Brentford defence before chipping the ball into the box.
As the Brentford defenders hesitated, Dany N'Guessan had time to turn and fire a low shot past Ben Hamer.
Two minutes later Mark Phillips headed the ball into the Lincoln net from close in but his effort was ruled out for offside.
But the Bees kept pushing forward and levelled on 13 minutes when Marcus Bean burst through the middle before firing a shot under the body of Lincoln's Rob Burch.
Brentford then went in front with a move started by keeper Hamer.
His 22nd minute free-kick over the top of the Lincoln defence found Dean Bowditch, who brushed off the challenge of Danny Hone before smashing his shot into the net.
Lincoln tried to get back in the game with both Stefan Oakes and Anthony Elding going close.
But seven minutes before the half-time break the home team were reduced to ten men when defender Green was sent out for a two-footed challenge on Brentford skipper Kevin O'Connor.
Brentford had chances to increase their lead with Glenn Poole twice being denied by Burch.
The Bees missed a glorious chance in the 59th minute when Bowditch got the right flank and sent in a low cross which found Charlie MacDonald unmarked.
But, from three yards out with an empty net in front of him, the striker somehow lifted his shot over the bar.
Lincoln, inspired by Horsfield, hit back and in the 66th minute the former Sheffield United striker flicked the ball on for Elding, who beat Hamer with a first-time shot.
Lincoln kept up the pressure and almost snatched a late winner. A 20-yard shot from Lee Frecklington was heading for a top corner, only for Hamer to make a superb one-handed save.