Serious Heart Condition Threatens Swamp People Star

It's not Troy ... or Trapper Joe ... or the badass Guist Brothers -- it's Bruce Mitchell's faithful sidekick Tyler ... the lovable gator hunting dog.

TMZ spoke to a rep for Bruce -- the guy who rocks the overalls and the American flag bandana -- who tells us Tyler was having "heart troubles" back in January ... and had to be rushed to the vet for emergency medical care.