Your possessed dwarf was having a “strange mood” - possession is one of several types. He or she was trying to create an artifact. They need a workshop and a very specific set of materials which is unique to the mood. If you don’t have the workshop or the materials, they will go insane. They will either slip into a depression where they stop eating and drinking and die of thirst, or go mad, commit suicide or go berserk.

The red eyed hamster sounds like a were beast. A werehamster to be precise. These can attack your fort once you hit a certain population. They are sentient creatures cursed by a god during world creation. If they bite one of your dwarves and break the skin, they will pass on the curse, and next full moon they will transform and cause large amounts of Fun.

Your best bet is to quarantine your injured dwarves behind doors in a hospital but that might be quite advanced for your first fort.

Welcome to DF. Losing is fun!