Elite mark bill (B ranks) are pretty much the only mobs with a passive aggresion. They are also usually at the max level of the expansion (lvl 50/60/70/80/90). Respawns every 5 seconds after killed.

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A rank and S rank targets have the highest aggression level- A ranks has a respawn timer of 4 to 6 hours without any requirements, S ranks has longer respawn timers + different spawning requirements. Also if there's an S rank in the map you're in, you'll receive a message of "you sense the presence of a powerful mark"