The 28-year-old will remain at Sixfields until the end of the season.
"With the loan window shutting, we needed cover in that position," Wilder told BBC Radio Northampton.
"[Current left-back] Gregor Robertson can play at left-centre-half as he's done a few times and done really well."
Leon McSweeney, Chris Hackett and Brennan Dickenson are all in contention to start against AFC Wimbledon on Tuesday night along with Horwood.