paste can also make it:
edited
This prints the first line again. Taking advantage of the lack of quotes, the trailing spaces are not showing:
$ paste -d"\t" <(printf "\n\n\n%s" $(head -1 file)) file
CAT
DOG DOG DOG DOG DOG DOG DOG
DOG DOG DOG DOG DOG DOG DOG
CAT DOG DOG DOG DOG DOG DOG DOG
DOG DOG DOG DOG DOG DOG DOG
original
$ -d"\t" <(printf "\n\n\nCAT") file
CAT
DOG DOG DOG DOG DOG DOG DOG
DOG DOG DOG DOG DOG DOG DOG
CAT DOG DOG DOG DOG DOG DOG DOG
DOG DOG DOG DOG DOG DOG DOG