Starting from a brief theoretical approach to idioms, the present article aims at establishing, using mathematical methods, the percentage of positive, negative and neutral idioms including the nouns ’cat’, ’dog’, ’horse’ and ’ox/bull’ in English and Romanian. In this comparativecontrastive approach reference will be made to a series of aspects allowing the analyzed idioms to be classified as semantically positive, negative or neutral. Special attention will be devoted to the negative idioms including the four nouns denoting domestic animals due to the fact that they are the majority. The negativity of those idioms is given not only by the negative words inside of them (they are, anyway, just a few), but also by their meaning (which is the case of the majority).