"This is a mouse."
Translation:Hic est mus.
Though there can be ambiguity in Latin, this is not very ambiguous--"hic" is usually translated as "this". The word order is fairly clear, also. What is ambiguous it that it could be "This is a mouse" and This is the mouse."--though, in practice, they had phrasings which could make the different meanings clear.
[Maybe "Res (the thing) est hic mus" would be your second sentence, though I think there are far more elegant Latin phrasings which do not come to my mind right away. Not sure what "it" would be in that sentence--that would determine the word and gender.]