The "dislike" button is one of those things that YouTube thought would help but ultimately ended up becoming an undesirable feature. In particular, swathes of users used it to launch campaigns to bring down select videos in a concerted and malicious effort. Because of this, YouTube decided to remove the "dislike" count on YouTube videos in an attempt to curb such plans. However, it appears that YouTube did not anticipate clever users who had the means to create browser extensions to bring the "dislike" counter back.
"Return YouTube Dislike" does exactly what the name says
In what has to be one of the most appropriately named software available, "Return YouTube Dislike" is an open-source software by Dmitry Selivanov. You can access its source code on GitHub and check it out yourself. Of course, for the less tech-savvy users out there, all that you need to do is that it is software that you can download for free and use as a browser extension for Chrome and Firefox as well as other browsers and even jailbroken iOS devices. Although it's still in its early alpha version, the extension works exactly how the name suggests and has already received a perfect 5-star rating from thousands of users.
