What cat does Barney refer to during your first meeting with Kleiner in Half life 2?

The cat did not appear in any of these games:

• Half Life (Source)
• Blue Shift
• Opposing Force
• Decay
• Either of the Portals
• Half Life 2 or anything released thereafter.

I'm going to call this a noodle incident (TV Tropes link, you have been warned).