Spawn blocking is fixed! Ok, sorta fixed. If you tame something and stay on server, it still counts as being part of the spawn pool. Once you take it off server, it is forever removed from the spawn pool. So now you can ride your nightmare inside dungeons without fear of ever blocking the spawn.
Spawns in general were changed. Some areas, like northeast of Minoc, were nearly void of animals and monsters. Now they are full of them, more animals than monsters. Horses have also been added to the animal spawns in the old lands.
Nightmares and white wyrms now only spawn in one body style. The white wyrms now only spawn with the bumpy skin that appears grey and white. Nightmares now only spawn with short manes. This change was done in preparation for UO:3D. Nightmares used to be horses that were "stained" a black hue. In the UO:3D client, they are their own animation. The spawns had to be adjusted for this, which is the reason for the removal of the other body styles of nightmares.