Why is "This duck" incorrect?
because seo at the start of the sentence like this is acting as an identifier. Grammatically there is an assumed 'S e which is not there either in speech or writing. Therefore, this phrase says "this is a duck".
This duck would be an tunnag seo
Tapadh leat! :-)
By the way, I saw you asked a number of questions on these sentences last night. I'm not sure if you've seen them, but I've responded to the ones I've seen.