Depressed Pug

Question
We have a 7-year male old Pug who has a wonderful, very typical Pug personality (great with kids, a little stubborn, playful, loves food). This weekend we are dog-sitting a neighbor's dog (a 2-year female Puggle) who also has a great personality. We thought the dogs would get along well but it turns out that our Pug is acting extremely depressed and intimidated by the Puggle (who has not been aggressive toward the Pug).  The Puggle will be returning home on Monday and then things will be 'back to normal'.  Is it typical that a dog would become depressed in this type of situation?  While we have shown our Pug lots and lots of attention this weekend (and even taken him on a few car trips to perk him up), it is possible that he thinks he's being replaced?  Will he get back to normal once the Puggle leaves?  

P.S. He just had his annual check up last week and got a perfect bill of health (plus he has been eating and drinking this weekend).

Thanks so much for any insight you can give--we feel so bad for our Pug!

Answer
It's impossible for me to see what you describe as "depressed" or "intimidated".  You MIGHT be seeing the typical reaction of an "only dog" to an interloper.  Even high ranking dogs in multiple dog households will persistently ignore a newcomer, refusing interaction, for quite a while in order to make a rank statement.  If your Pug IS experiencing depression and if he is slinking around and very obviously intimidated/fearful of the other dog, this is not a situation you want to repeat!  Your dog should return to normal after a few days following the disappearance of the visitor.  Don't expect it to happen immediately, it might take a few days.  Treat your dog normally and interact with him as you usually do and everything should return to normal.