Boyd scored in the 16th minute thanks to a run behind the Seattle defense and an accurate cross from Steven Smith, putting the ball into an empty net.
Horst doubled the lead 9 minutes later when he headed a Franck Songo'o corner kick into the net.
The Timbers (4-6-4) came up with just enough defense in the second half to notch their first MLS victory over bitter rival Seattle.
The Sounders (7-5-4) got a 58th-minute goal from Eddie Johnson but it wasn't enough to prevent their club-record winless streak in league play from reaching seven games.
The game erupted into fights during stoppage time, when Seattle's Fredy Montero was given a red card for leveling Horst. Also ejected was Portland defender Lovel Palmer, who elbowed Johnson during the scuffle.
Justin Bieber crashes Drake Bell's album release party
Members of Congress to keep receiving porn magazine