During my last several years in Bethel, I got to know most of the Art Department since I was VERY close to them... that's all I'll say. Their lives revolved around production and it could be very frustrating for them. From the painting/drawing side, they would be given much more time to do artwork for books. Often, years are spent on writing and design. Consequently, the Revelation book has a superior, much more accurate illustration.
The Watchtower, however, appears to have been done by one of the lesser experienced artists, particularly at horses. For the artists, they have a large but limited number of images to search through to use as a basis for the painting. I would guess the designer wanted a new piece done for the new light page, rather than recycle existing artwork. Artist A is busy on the next book, etc. So, they go all the way down to artist Q. He does it. The designer says that it needs to be narrower and more aggressive looking. Patch job rushed for deadline. Artist scrambles to meet deadline. Picture suffers.
Yes, left front leg appears to bend the wrong way forward. Horses head doesn't look right at all. I've seen lots of horses and that is one butt ugly horse. Proportions of horses (ponies) and riders and depth of action is out of whack. The dude on the red horse appears to be almost as large as JC although the attempted perspective of the picture puts him well behind. Either the red dude is huge, or the artist goofed. Also, the direction of the light source on the subjects is not consistent. And kinda like Oompa points out, the red dude would make a way awesomer Jesus than the white horseman in the foreground. JC looks wimpy and on a sickly looking horse. Hey, wait, I think they used Ted Jaracz as the Jesus model and Melita as the horse... http://www.jehovahs-witness.com/12/148974/1.ashx
All in all, it's not the worst piece to come from Art Department. I think it was the True Peace book that had several that were absolutely craptastic!
Anyway, no secret messages of Satanic horses to my knowledge, just rushed and somewhat inexperienced artists trying to keep up with the JW mantra "DO MORE, THE END IS NEAR!! DO MORE, THE END IS NEAR!!..."