JSFiddle - React, Tailwind, and code Playground

JavaScript

var farm = [];
farm.push(['Cats', 3], ['Cats', 4], ['Mice', 2], ['Dogs', 5]);

var o = {};

for (var i=farm.length; i--;) {
    var key = farm[i][0],
        val = farm[i][1];
    
    o[key] = key in o ? o[key] + val : val;
}

var list = '';

for (var key in o) {
    list += key + " (" + o[key] + ")\n";
}

console.log(list)