What does Fast Down Button look like on HTC?

On HTC it is the design shipped in Sense 8, shown above at full size. Older revisions are listed beneath it.

When did HTC add Fast Down Button?

HTC first drew Fast Down Button in Sense 7, released March 30, 2015.

HTC Corporation has shipped 2 versions of Fast Down Button. Each is dated, so you can see when the design changed.

HTC

HTC Corporation has drawn Fast Down Button 2 times. Every revision HTC has shipped is below, newest first.

What does Fast Down Button look like on HTC?: On HTC it is the design shipped in Sense 8, shown above at full size. Older revisions are listed beneath it.
When did HTC add Fast Down Button?: HTC first drew Fast Down Button in Sense 7, released March 30, 2015.
How many times has HTC redrawn Fast Down Button?: HTC Corporation has shipped 2 versions of Fast Down Button. Each is dated, so you can see when the design changed.

HTC artwork for this emoji spans 2015 to 2016.