[kscherer]

It's all about the area around the portal and how much room the animal has when they spawn. Quite often they will spawn with their butts out the entrance and their heads in the wall. They quickly suffocate and die. Portals and animals are tricky business.

Jess figured this all out and we now have a horse-roundup/corral in the Nether. I moved about 30 horses a few weeks ago and her and I just moved another 16 last night.

All portals involved need to have a hollow backside, i.e. at least one empty block on the non-exit side of the portal so the animal's head can safely spawn without getting "lopped off". Preferably, all portals involved should have empty space all the way around of at least 3 blocks.

The OW portal I use at the roundup has nothing blocking either entrance. I back through (past) the portal veil—such that I don't go into the Nether—while pulling the horse right up to the veil. As soon as the horse disappears, I follow it through, lasso it, and maneuver it to a holding pen before going back for more. To get them out, it is roughly the opposite. Jess and I will also push the horses through and tag-team with one of us on each end of the portal. When moving a large number of animals, working in teams is far easier.

Either way, of the 50+ horses we have moved in the past few weeks, we have only lost 2.

If you want to see it in action, you are welcome to visit our roundup at the very far end of the East tunnel (just before the Wither Skellie spawner)