Please don't take this the wrong way - it might be a bug - but we've had a couple of similar reports from other
Lumion 1 users, and both times the disappearing models were caused by the fact that they were using a "cracked" version of Lumion. By "cracked", I mean a version which consisted of a mixture of
Lumion Free 1 and
Lumion Ultimate 1. Unfortunately, some people don't seem to realise that they are using a "cracked" version, as they have downloaded it from a random website.
Again, I'm sorry to have to be this cautious, but to avoid wasting both your time and my time, I have to be 100% sure that you are using a squeeky clean version of
Lumion Free which was downloaded from the Lumion website

Also, it would be helpful if you could save your
Lumion Free 1 scene, install
Lumion Free 2 and let me know if you can see the horses in that version?