Answer: dog

AnswerCrossword Clue
DOGStay close behind
DOGGuard or show follower
DOGKennel denizen
DOGRetriever, e.g.
DOGLab or peke
DOGLoaf, with "it"
DOG"Man's best friend"
DOGUnderperforming investment, slangily
DOGFollow persistently
DOGHound
DOGLab, for one
DOGLucky one?
DOGFetch player
DOGBoxer or pug
DOGCat chaser
DOGWorthless thing, in slang
DOGFlop
DOGFollow
DOGBall game treat
DOGTail tirelessly
dogdiron
DOGBoxer, for one
DOGJack Russell, for one
DOGPluto or Lassie
DOGPug or boxer
DOGGo after
DOGOdie
DOGDrug-sniffer, perhaps
DOGFrequent "Too Cute!" animal
DOGPooch
DOGFollow closely
DOGFido, e.g.
DOGTrack relentlessly
DOGBother persistently
DOGFollow relentlessly
DOGTrail persistently
DOGSubject of Pavlov's research?
DOGBone exhumer, possibly
DOGChinese-zodiac animal
DOGPursue relentlessly