JSFiddle - React, Tailwind, and code Playground

JavaScript

var farm = [['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;
}

console.log(o)