As I plow the older topics on this forum, I stumbled upon this claim and would like to respectfully correct you and Voiou.
The word "kuce" (куче) is Proto Indo-European (PIE) whose etymology is in the word "kukkura":
http://vedabase.net/k/kukkura
So "kuce" is not an original Bulgar word.
Neither is the word "bubrek" (or bubreg in Macedonian) which means kidney but also a mountain.
In Macedonia, nobody used to say "kuce" before 1918, that is a Serbian import. As stated, Macedonians traditionally use "pes".
Best regards,
M