Neopets userscript, very early version probably. Tested with https://github.com/janekptacijarabaci/greasemonkey/releases/tag/3.31.4Fork on Pale Moon.

Old Pets.js

// ==UserScript==
// @name        Old Pets (Sidebar/lookups, config needed)
// @namespace   neopets.com
// @include     http://www.neopets.com/*
// @version     1
// @grant       none
// ==/UserScript==

// Here's the manual bit, in case you want the right color to show up instead of just yellow. Add an entry for each pet name.
// Keep it lowercase for the sake of convenience.
var pets = {
  'beocitiesthesecond': 'yellow',
  'belmari': 'red',
  'bzward': 'green'
} // (Basic, easier to configure-ish but doesn't show on lookups and stuff. If you do the one below you don't need this.)

// Advanced, but goes further across the site; replace certain image URLs with the old ones. 
// Key is the current pose URL, value is species_color.

var images = {
  'http://pets.neopets.com/cp/o5ml3qwc/1/2.png': 'kougra_red',
  'http://pets.neopets.com/cp/sc2hhvhn/1/2.png': 'bori_yellow',
  'http://pets.neopets.com/cp/schfx5c4/1/2.png': 'buzz_green'
}

// (config ends)
var all = document.getElementsByTagName('img')
for (var i = 0, max = all.length; i < max; i++) {
  if (all[i].src in images) {
    console.log(all[i].src)
    var img = 'http://images.neopets.com/pets/happy/' + images[all[i].src] + '_baby.gif'
    console.log(img)
    all[i].src = img
  }
}

// Do sidebar stuff
var colour = 'yellow';  // I'm only using the spelling because the site's Welsh I swear.

var name = document.querySelector('td.sidebarHeader > a > b').innerHTML.toLowerCase()
try {
  colour = pets[name];
}
catch(error) {
  document.title = document.title + ' [Unknown pet]' // this doesn't work atm
  console.error(error);
}

var species = document.querySelector('td.activePetInfo > table > tbody > tr > td[align="left"] > b').innerHTML.toLowerCase()

var image = document.querySelector('td.activePet > a > img')
image.src = 'http://images.neopets.com/pets/happy/' + species + '_' + colour + '_baby.gif'
image.width = 150