From version 3.3, the built-in ad blocker is retired. Horse runs uBlock Origin Lite instead, which does the job better than a copy of it ever would. This page describes the built-in blocker as it works up to 3.2.

Horse Browser ships with the ad blocker on. There’s no extension to install and no third-party app to grant permissions to, it’s part of the browser. You can turn it off any time you want; that’s mostly what this page covers.

A reason you might turn it off: a site you’re trying to support, or a site whose layout breaks without the ads it expects.