Short legs & probably the 53mm spaced bridge pickup (all neck models are 49mm).
Although it has a Tune-o-matic, the Horizons (& other ESPs) I've played still had the string spacing quite wide. To be sure, get a ruler & measure the distance between the top & bottom strings over the existing bridge pickup pole pieces, then buy whichever spacing is nearest.
In reality it's not a particularly big deal as long as the strings pass over some portion of the pole pieces - most guitars could quite happily take either. The total dimensions of the pickup are the same whichever you choose, so it will always fit.