Pure and impure animals
Question

Are there any guidelines to determine which animals are impure (najis) in and of themselves and which are pure (tahir) in and of themselves? Are crocodiles impure in and of themselves?
Praise be to Allah.There is scholarly consensus on the purity of some animals and the impurity of others, and there are some concerning which there is a difference of scholarly opinion.
Animals whose flesh may be eaten are pure according to scholarly consensus. That includes the an‘am animals (camels, cattle and sheep). That also includes birds which may be eaten, such as pigeons and geese, and also sea creatures such as fish of various types, except for a few concerning which there is a difference of scholarly opinion, such as crocodiles. Those scholars who think that it is permissible to eat them also regard them as pure.
As for animals whose flesh may not be eaten, such as predators and domesticated donkeys, there is a difference of scholarly opinion as to whether they are pure or impure.
Ibn Qudamah (may Allah have mercy on him) discussed the issue of animals and whether they are pure or impure, and the types concerning which there is scholarly consensus and those concerning which there is a difference of scholarly opinion, in a very smart way. There follows a summary of what he said.
He (may Allah have mercy on him) said: With regard to leftover water and animals there are two categories: impure and pure.
Those which are impure are of two types: the first type is that which is impure according to all scholars, namely dogs and pigs, and that which is born from both of them or one of them. They are impure in and of themselves, and their leftover drink and anything that comes out of them is also impure. That was narrated from ‘Urwah and it is the view of ash-Shafa‘i and Abu ‘Ubayd, and it is the view of Abu Hanifah regarding leftover water in particular.
However, Malik, al-Awza‘i and Dawud said: Their leftover water is pure, and it may be used for wudu and drunk. If they eat from some food, it does not become haram to eat it.
The second type are those concerning which there was a difference of scholarly opinion [as to whether they are impure]. This type includes all predators, except cats and creatures that are smaller than cats. It also includes birds of prey, donkeys and mules. It was narrated from Ahmad that their leftover water is impure, and if nothing else can be found, one should do tayammum and not use it for wudu. It was narrated from Ibn ‘Umar that he regarded the leftover water of donkeys as disliked (makruh). This is also the view of al-Hasan, Ibn Sirin, ash-Sha‘bi, al-Awza‘i, Hammad and Ishaq.
It was narrated from Ahmad (may Allah have mercy on him) that he said regarding mules and donkeys: If there is no water except their leftover water, one may use it for wudu and also do tayammum. This is also the view of Abu Hanifah and ath-Thawri. This report indicates that their leftover water is pure, because if it were impure, it would not be permissible to do wudu with it. It was narrated from Isma‘il ibn Sa‘id: There is nothing wrong with the leftover water of predators, because ‘Umar said regarding predators: We often encounter them. Al-Hasan, ‘Ata’, az-Zuhri, Yahya al-Ansari, Bukayr ibn al-Ashajj, Rabi‘ah, Abu’z-Zinaad, Malik, ash-Shafa‘i and Ibn al-Mundhir granted a concession regarding the leftover water of predators.
The correct view in my opinion is that mules and donkeys are pure, because the Prophet (blessings and peace of Allah be upon him) rode them, and they were ridden at his time and at the time of the Companions. If they were impure, the Prophet (blessings and peace of Allah be upon him) would have pointed that out. Furthermore, they cannot be avoided, for those who keep them, so their case is similar to that of cats. The words of the Prophet (blessings and peace of Allah be upon him), “They are an abomination (rijs)” mean that they are haram to eat, just as Allah, may He be exalted, says of alcohol, gambling, stone altars [for sacrifices to idols] and divining arrows that they are {an abomination (rijs)} [al-Ma’idah 5:90]. And it may be that when he said that, he was referring to the flesh of donkeys that was in their cooking pots, which he described as an abomination. So if they are slaughtered it is not permissible to eat them, because slaughtering them does not make them pure.
The second category of animals is those which are pure in and of themselves, and their leftover water and sweat are also pure. They are of three types:
The first type is human beings. The human being is pure and his leftover water is pure, whether he is a Muslim or non-Muslim, according to most of the scholars. But it was narrated from an-Nakha‘i that he regarded the leftover water of a menstruating woman as disliked (makruh).
The second type is animals whose flesh may be eaten. Abu Bakr Ibn al-Mundhir said: The scholars are unanimously agreed that the leftover water of animals whose flesh may be eaten is permissible to drink and use for wudu.
The third type is cats and smaller creatures, such as mice and rats, and weasels. The leftover water of these and similar creatures, such as the insects of the earth, is pure and it is permissible to drink it and use it for wudu; that is not disliked. This is the view of the majority of scholars among the Companions, the Tabi‘in and the scholars of Madinah and Syria, and the scholars of Kufah, who are ashab ar-ra’y, except Abu Hanifah; he regarded it as disliked to use the leftover water of a cat for wudu, but if that is done, it is valid."(Al-Mughni  1/43-45).