Ayesh Karunaratne

Fast 404 is a low foot-print plugin that quickly inspects an incoming HTTP request, and terminates the request as soon as possible if the request is for a non-existing resource. If the browser is expecting an HTML page (indicated by the <code>Accept</code> HTTP header), this plugin will not intercept it. For all other requests, this plugin will terminate it immediately, saving server resources and bandwidth.
When a user browser requests a resource (such as a <code>jpg</code> image, or a <code>.woff2</code> font file), the web server sends this resource if it is available in the requested location. If the file does not exist, the request is forwarded to WordPress to handle. Unless you are using a plugin that dynamically generates these files, these file-not-found requests trigger a full WordPress Page-Not-Found error page. This plugin inspects such incoming requests, and if the browser indicates that it is looking for a resource other than an HTML page, this plugin terminates the request as soon as possible to prevent WordPress from serving this request which would be a waste of resources and bandwidth. This plugin carefully makes sure that the short-circuited 404 pages (which just shows “Not Found” on a blank page) is only returned to browser asset requests, and not for end users who expect an HTML page.
By default, all HTTP requests to <code>js|css|jpg|jpeg|gif|png|webp|ico|exe|bin|dmg|woff|woff2</code> extensions will be fast 404’d. You can configure the extensions and even configure an exclusion pattern to prevent this plugin from intercepting certain requests.
This plugin is the WordPress port of <a href="https://github.com/PHPWatch/Fast404" rel="nofollow ugc">PHPWatch/Fast404</a> package.

Fast 404

Prevents WordPress from delivering full Page-Not-Found errors when the browser is not expecting a full HTML page. Saves bandwidth and improves perform …

brohar

PF404 for PetFinder will replace your generic 404 page and replace it with animals available for adoption. You can select between dogs, cats, birds, reptiles, horses, small and fury, and even barnyard animals. Configure the page heading and contents of your 404 pages within the plugin settings page. 
This plugin does require a free PetFinder API key. 
This plugin or plugin developer is not affiliated with PetFinder.com.

PF404 for PetFinder

Overrides your 404 page template and shows dogs, cats, and other animals in need of new homes and available for adoption.

Fast 404 vs PF404 for PetFinder