Interest (riba) is haraam and is a major sin; it is not permissible to deal in it even if the one who does so intends not to benefit from it himself, rather to spend it on charitable causes.
Shaykh Ibn Baaz (may Allah have mercy on him) was asked:
Is it permissible to put money in the bank in order to take muraabahah on it, knowing that my aim in getting this profit is to distribute it to charitable causes and so that this money will not remain idle?
He replied: It is not permissible to do that, because this is the essence of riba which the scholars have stated is not allowed and which the texts of the Sunnah indicate is haraam. This is what is called a loan that brings benefits; what is meant is the stipulated or agreed-upon benefit. Even if the one who does that has a good intention, (it is still forbidden) because Allah has forbidden riba because of the corruption and harm it causes to society as a whole and to the poor in particular. The Muslim can this money to the bank or some other agency with the aim of investing it in Islamic ways such as mudaarabah. End quote.
Majmoo‘ Fataawa Ibn Baaz, 19/134
Whoever falls into haraam, riba-based transactions and wants to repent from that has to get rid of this haraam interest by spending it on the poor and needy or on charitable projects that serve the common interest, such as hospitals, schools, orphanages and so on.
The scholars of the Standing Committee for Issuing Fatwas said:
It is essential to get rid of the bank interest because it is a type of haraam riba. So it should be spent on the public interests of the Muslims, such as giving it to the poor and needy, in order to get rid of it. End quote.
Fataawa al-Lajnah al-Daa’imah, 16/532
Based on that, if a Muslim is forced to take interest or he has repented from it, then he has to spend this money on charitable causes. If he wants to spend it on pets or other animals, if this spending comes under the heading of kindness and compassion towards them because they need food, that is permissible because that comes under the general meaning of kindness. Al-Bukhaari (2363) and Muslim (2244) narrated from Abu Hurayrah (may Allah be pleased with him): They said: O Messenger of Allaah, will we have reward with regard to these animals? He said: “In every living thing there is reward.”
Al-Nawawi (may Allah have mercy on him) said:
This hadeeth encourages kindness towards “respected” animals, which are those that we are not enjoined to kill. So reward may be attained by giving them water and treating them kindly, as well as feeding them and other actions, whether they are owned or free, and whether they belong to you or someone else. End quote.
But if spending on them is by way of extravagance and excessive care of pets, as is the case in the West and among those Muslims who imitate them, that is not permissible, because that is a kind of foolishness, extravagance and waste of money.
It is better to spend this money on public and private interests of the Muslims, especially if there are many Muslims who are in need and have no one to spend on them.
And Allah knows best.