In ancient forest when you fight the big dragon boss. After boss fight I went with portal to sell my loot. After that I came back and boss was respawned. I thought oh, so bosses got a respawn timer? I killed it again for the second time, this time I went by portal again, but didnt waste any time in town so went right back to the area where he spawns again by portal and he was right there respawned again. This might be possible with other bosses? Haven’t tried yet.
Boss name: Primeval Dragon
Just wanted to let you know.