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0009882Dwarf FortressAdventure Mode -- Combatpublic2016-07-02 02:442016-07-09 23:28
normalminorhave not tried
Desktop PCWindows 7
0009882: Trained Megabeast (Dragon) Hostile in Visited Retired Fort
I caught a dragon in my fort and kept him trained, plus war training, in a wooden cage. I retired the fort and started a new adventurer as a newly minted fortress guard (hearthperson) of the site. I set the moss below cage on fire and wandered around the fort a bit.

When I returned, the Stray War Dragon was loose (as planned) and burning down everything and anyone in sight (not planned.) The "Stray War" title disappears in fort mode when a creature reverts to wild, so I assume it's not that.

I don't have other megabeasts to try with, but it might affect them too. I didn't try with my Giant Cave Spiders because they're in an artifact cage on stone floor. It might be a general issue with trained wild animals.
No tags attached.
related to 0010731acknowledged Loci Fully domesticated megabeasts and active military dwarves attack each other on sight. 
Issue History
2016-07-02 02:44BumberNew Issue
2016-07-03 01:50InfantIguanaNote Added: 0035551
2016-07-09 23:28BumberNote Added: 0035589
2018-05-01 07:20LociRelationship addedrelated to 0010731

2016-07-03 01:50   
This might be a stretch, but this could be related to 0009641.

If you tried removing the [MEGABEAST] tag from the dragon's raws in your save, I'm willing to bet it wouldn't exhibit this issue. The tag tends to make megabeasts hostile in certain situations, regardless of their "trained" status.
2016-07-09 23:28   
It seemed okay in fort mode for the time I had it pastured. But, yeah, I suspect the megabeast tag.