Is the lachan missing an I? I thought ducks should be lachain?
Yes, it should.
I think I remember reading somewhere else that both plurals exist...
lachan is the genitive singular and plural, whereas lachain is the nominative. They would never be used (or pronounced) in the same way.
Duh. They're horses.
I believe them.
When do you use iad instead of siad? I read that iad is usually used for "them," but in this sentence I wouldn't say "Them aren't ducks."
You only use siad (and sé and sí) when they are the subject of, and immediately adjacent to, a verb.
It's still incorrectly flagging 'lachain' as a typo for the nominative plural.
I'm surprised that hasn't been fixed by now.