"The puppy is growing."
A puppy is really a "koiranpentu". "Pentu" is a cub of mammals in general.
"Koiranpentu" does seem to be an option that would be more useful and have more clarity than just 'pentu' (which is causing some consternation across this section's forums!). I would have thought the translation of 'pentu' is more akin to 'young', as in the catch-all word for young animals plural: "The fox's den contained five young"; "The young of the hedgehog have soft spines". We wouldn't put "The young is growing", which it seems the Finns do. Why so unspecific, Finland?!