Is it correct that "This is a lion" is not accepted? How would I say that then?
I asked a dane about it and she said that "this is a lion" is also correct. 'Det' can means both things.
However, duolingo is probably suggesting 'that is a lion' as the correct form because you wouldn't be able to say 'this is a lion' close to a lion that has a notorious fame of eating people (as we learn in many exercises) :P
'This is a lion' would either be 'Dette er en løve' or 'Det her er en løve' (more colloquial). Det only means 'it' or 'that'