One sign of an animal being evil that the Prophet sallAllahu aalyhi wa sallam gave was that a people could have been punished by having been transformed [Maskh] into the form of animal. This indicates that the particular animal is evil by its very nature as Divine wrath transformed it into an animal.
For example in Surah al-Ma’idah ayah 60, Allah subhanahu wa ta’ala says,
قُلْ هَلْ أُنَبِّئُكُمْ بِشَرٍّ مِّن ذلِكَ مَثُوبَةً عِندَ اللَّهِ مَن لَّعَنَهُ اللَّهُ وَغَضِبَ عَلَيْهِ وَجَعَلَ مِنْهُمُ الْقِرَدَةَ وَالْخَنَازِيرَ وَعَبَدَ الطَّـغُوتَ
“Say, ‘Shall I inform you of [what is] worse than that as penalty from Allah? [It is that of] those whom Allah has cursed and with whom He became angry and made of them apes and pigs and slaves of Taghoot. Those are worse in position and further astray from the sound way.’”
This proves that both these kinds of animals are evil by nature – they, even if slaughtered, would still not become lawful. Then there are animals whose very doings and likings are sufficient for people of natural disposition to sense the evil inside them. For instance, there are beasts who live by injuring, tearing and eating other animals – a hard-hearted lifestyle indeed.
Similarly, there are many animals among crawlers and fliers whose very nature is to hurt or snatch away things, for instance, the snake, scorpion, house-lizard, fly, a falcon and others like them. So, the Prophet sallAllahu aalyhi wa sallam set the rule that animals among beasts which tear animals apart with teeth, such as the lion and the wolf and others of their kind, and animals among birds such as the falcon and the hawk and others of their kind which hunt with their claws, are all unlawful; or animals which are by nature mean and low or get themselves befouled with impurities, such as the rat or animals which eat carrion, or the donkey and similar others, all fall under the category of animals the physical properties and harmfulness.