YouTube has added a new feature for content creators: they can now add a 3-second intro to all their videos. The intro will automatically start on videos on their channel.
To set it up, you need to upload the 3-second introductory video as an unlisted video, and then select "add a channel branding intro" on your channel's InVideo Programming page. Then you can choose which videos the intro will appear on.
YouTube notes that the intros may not be used as ads, sponsorships, or product placements -- they're mainly a tool to help you promote your brand.