Animal Anatomy (one) – The character can only turn into one type of animal.
Gorilla-Man (Marvel) turned into a Gorilla and retained his intelligence but cannot turn back into a human (Agents of Atlas V1 #2).
In Tales to Astonish V1 #48, a scientist turns into a gorilla but also loses his intelligence.
Matthew the Raven (Vertigo) is a man who has been turned into a raven (Who’s Who in the DC Universe #15).
Vampires generally can only turn into bats.
Ursa Major (Marvel) can turn into a bear (Official Handbook of the Marvel Universe V1 #11).
Wolfsbane (Marvel) can turn into a wolf (Official Handbook of the Marvel Universe V1 #12).