The dark purple/blue cat that is on the left side of the FallenCLan cave entrance.
Viewing post in West Woods: Over Borders comments
There are a few more steps - After you say no, talk to Moose. Give the wilted flower (can be cound to the left of the entrance to Sunclan camp) to Nightlight. Wait a few days and go in the tent once Moose disappears from outside the tent, then talk to Moose and Grasswhistle. After that, talk to Nightlight. She will be frustrated, but you will have gotten the achievement. I hope this helps!