"I am a cat."
I'm not sure if it is acceptable in Polish but strange, or just not acceptable, but Duolingo tips and notes are
There is, however, one crucial difference: you usually cannot use this construction with pronouns. Sentences such as On to chłopiec or Ona to dziewczynka sound rather sloppy.
If you wanted you could say "jestem kotką", (kotka is female cat)
Jestem kota means I am cat's