Flying and Swiming Mobs Spawn out of Map

I had a flying mob spawn out of the Borders of a Map. It was a Desert Map with a ledge on one side, like a view to the Ocean i guess. There was a Wasp rare mob flying above the ledge out of the usual borders of the map. I cant quite remember it it was able to fly into the map but i think i was just able to damage it somehow.
In another map, Grimhaven, a swimming mob, the giant gecko, spawned swimming outside the map. I could not reach him with any attacks or casts and since he was a rare mob the map was lost...
There are maybe borders where Flying and swimming mobs are allowed to spawn while walking ones can not, since i did not see a walking mob outside the map jet.
All the maps spawned outside were also rare.