tag-name-change.js

// ==UserScript==
// @name        Derpibooru Tag Name Change
// @include     https://derpibooru.org/*
// @include     https://trixiebooru.org/*
// @version     1.0.2
// @grant       none
// ==/UserScript==

const tags = {
  'busty applejack': 'applerack',
  'busty minuette': 'curvegate',
};

function replaceName(el) {
  if (tags.hasOwnProperty(el.textContent)) el.textContent = tags[el.textContent];
}

[...document.querySelectorAll('.tag__name')].forEach(replaceName);