Exiting Victoria Park along with Horwood are Nathan Buddle, Paul Johnson, Nathan Luscombe, Adam McHugh and Colin Nish.
But the club, relegated from npower League One this season, have exercised the option on the contracts to keep Neil Austin, Lewis Hawkins and James Poole.
A statement on Hartlepool's official website said: "The chairman and everyone at the club would like to thank all of the players for their services and commitment to the club and wish them all the very best for the future.
"Discussions are continuing with other remaining first-team players and more information will be made available in due course."
Hartlepool last week