Horsburgh's extension was announced by the Green Machine on Wednesday with the 25-year-old set to remain at the club until 2027.
The fiery forward joins Hudson Young, Joe Tapine and Seb Kris in committing to Canberra for the long haul.
"Having Corey re-sign with the club long-term allows us to continue to create some stability and consistency in our forward pack and he will continue to be a big part of the team into the future," said Raiders recruitment manager Joel Carbone.
"Corey has developed into a representative player here at the Canberra Raiders and we know he still has a lot of improvement in his game; we expect to see the best of his talent over the next four years."
Canberra's big off-season objective is to sign a five-eighth to replace Jack Wighton, who has left to join South Sydney.
An edge back-rower is also a target given Elliott Whitehead's age and concerns over Corey Harawira-Naera, who hasn't played NRL since convulsing on the field in a win over the Rabbitohs.Â
Elsewhere, Manly have secured the services of Wests Tigers prop Aitasi James on a two-year deal.
"Aitasi reminds me of Junior Tatola when he came to South Sydney from the Tigers," said Manly coach Anthony Seibold.
"He has a similar body shape and is a similar type of player.
"He has got that potential already. We are giving him an avenue to try and reach that potential."
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