Horizon went out of business a few years back. I think over-extending on the licesensing, and recasts/bootlegs of their kits really hurt them.
Their last few kits were from their Dinosaur line. They were beautiful kits, and HUGE, but I think it was too little too late.
Originals of most of the kits can still be found at decent prices on Ebay. The ones that seem to command high dollar are:
Jurassic Park T-rex, and Raptor
Stargate Resin kits
Windslasher original vinyl kit
ED-209
Mecha Battletech Droid
I would steer clear of any of the vinyl recasts from Thailand, or elsewhere in Asia. They are inferior copies.
Charlie