Despite an earlier report, it turns out that Philadelphia Eagles defensive end Fletcher Cox was NOT fined for a horse collar tackle. He was, however, fined $16,537 for a hit on Jacksonville quarterback Chad Henne that took place in the fourth quarter of the Eagles-Jaguars Week 1 game.
The hit is pictured in the photo above this article. Here's a GIF of the play.
It's hard to clearly see in the GIF but Cox does make illegal contact with his helmet to Henne's helmet and/or neck area. Here's the rule Cox broke, straight from the NFL rulebook.
In covering the passer position, Referees will be particularly alert to fouls in which defenders impermissibly use the helmet and/or facemask to hit the passer, or use hands, arms, or other parts of the body to hit the passer forcibly in the head or neck area (see also the other unnecessary roughness rules covering these subjects). A defensive player must not use his helmet against a passer who is in a defenseless posture — for example, (1) forcibly hitting the passer’s head or neck area with the helmet or face mask, even if the initial contact of the defender’s helmet or facemask is lower than the passer’s neck, and regardless of whether the defensive player also uses his arms to tackle the passer by encircling or grasping him; or (2) lowering the head and making forcible contact with the top/crown or forehead/"hairline" parts of the helmet against any part of the passer’s body. This rule does not prohibit incidental contact by the mask or non-crown parts of the helmet in the course of a conventional tackle on a passer.
Cox plans to appeal his fine but I'm not sure his attempt will be successful.
Cox wasn't flagged for hitting Henne, but the NFL obviously thinks he should have been. Ironically, Cox was flagged for the supposed horse collar tackle, but (correctly) not fined for it.