The New Zealand-born forward made 24 appearances last year, scoring five tries but has now opted to retire at the end of the campaign.
Last week, the Rugby Football League shared its annual list of Super League players that are out of contract at the end of 2019.
On the list, players' status are marked as 'out of contract' - which means they have been told they will not be offered a deal for next season - or as 'not yet determined,' denoting that the club has not decided on whether or not to offer new terms.
Of the 11 Trinity players currently out of contract at the end of the year, Horo is the only one that currently won't sign a new deal.
Horo joined Wakefield from Catalans Dragons ahead of the 2018 campaign, signing a two-year deal.
He scored the final try in Wakefield's sixth-round Challenge Cup victory over Widnes Vikings on Friday night.
Meanwhile, players who are out of contract and have been offered a deal by their current club or who have agreed a deal elsewhere have not been listed.
The only Wakefield player not listed is half-back Ryan Hampshire.
The not yet determined Wakefield players are: Chris Annakin, Mason Caton-Brown, Keegan Hirst, Luke Hooley, Craig Huby, Danny Kirmond, Pauli Pauli, Tyler Randell and Lewis Wray.