Gary Flowers and Maurice Bolden each had 16 points for the Golden Eagles (14-3, 3-1 Conference USA), who used a 21-2 run midway through the second half to turn a four-point deficit into a 68-53 lead.
Horton was terrific during the decisive stretch, hitting a 3-pointer, completing a conventional three-point play and throwing a perfect alley-oop pass for a thunderous dunk by Flowers.
UCF (14-2, 1-2) has lost two in a row after starting the season on a school-record 14-game winning streak. The Knights, who earned the first national ranking in program history on Dec. 20, will likely fall out of the polls.
Marcus Jordan, the son of NBA great Michael Jordan, scored 15 first-half points and finished with a team-high 20 for UCF.