This is the 12352nd most frequent German word.
vierbeiner
"Vierbeiner" means "four-legged animal."
Here, "Vierbeiner" is used as a noun to refer to an animal with four legs, specifically a friendly dog in this context.
Der Hund ist ein freundlicher Vierbeiner.
The dog is a friendly quadruped.
In this sentence, "Vierbeiner" is again a noun indicating animals with four legs, generalizing them as companions to humans.
Viele Vierbeiner sind zuverlässige Begleiter für den Menschen.
Many quadrupeds are reliable companions for humans.
Here, "Vierbeiner" describes the diverse four-legged animals that a veterinarian deals with, showcasing a professional context.