Back in 40d, I embarked on this great 4x4 map. Sand, Magma pipe, underground river, nice high cliffs, and tons of trees. As I soon found out, it also had a cave, wherein dwelled a cyclops. Never having encountered one before, I checked up on a few things. He wasn't termed "Hostile", but "Resident". I was confused, checked up on more things, and found out that it was this cyclops from the legends of my world, who had chosen to settle down in that area.
A year went by, and I noticed that the cyclops had moved from out of the cave to about fifty tiles out of the mouth. She sat there for a long time, which caused problems- my trappers, woodcutters, and project builders kept getting terrified and interrupting their jobs. So I go, "Well, she's not really causing any problems, but she's got to go. I assembled a squad of six elite wrestlers, and sent them out to deal with her.
She killed five, and the remaining one fled for his life. The cyclops gave chase, followed him inside, into the dining room, where she killed him and everyone eating, and just stood there. I watched warily. She was content, now that she couldn't see any dwarves to kill, to sit on a dining-room chair and just chill. I built walls around the doors, and carved out a new dining room.
The cyclops sat there for eight years. I sort of just forgot about her. When I realized that I could have two dining rooms, I carved some fortifications in the old walls and had my marksdwarves deal with her. I was a little sad afterward.
Also, my tamed alligator. Nobody wanted to adopt poor Roldethdustik.