HERE is the closest I could find to looking like him. Most of the pug/rat terriers I found were all under a year old, so they weren't filled out yet, Hopefully none get quite as filled out as my dog is. The dog in the link looks most like Frisk did at that age.

AND while on the quest to find Frisk's twin, I noticed that he's the oldest Pug/Rat terrier mix on dogster. =]