The first mod to ever add Herobrine to Minecraft was a hmod server plugin from December 2010 called "iloveyou" created by the forum user tkelly. Multiple versions of this plugin were released in December and the following January, but it seems to have stopped being updated after this. All of these versions of the plugin are lost.
A few others made their own modified versions for later updates such as beta 1.7, beta 1.8 and release 1.5, but only the 1.5 and 1.7 versions have been found.
This mod is significant not only for being the first of its kind but also because a lot of popular Herobrine tropes like the Herobrine summoning shrine and underground Herobrine tunnels lit by redstone torches were invented by this mod and popularized by YouTube videos showing it in action.
Since it was a server plugin, anyone who used it on their server (and there is video evidence of a lot of different servers using it) would have had to download it and may still have it downloaded. The original creator of the plugin and the creators of later versions may also have versions of it.
Here are some links to relevant webpages with evidence:
https://www.youtube.com/@tkellyMC
https://web.archive.org/web/20110720002711/http://topica-rp.com/hmod/forum/showthread3b9b.html
https://www.reddit.com/r/Minecraft/comments/ekq34/herobrine_plugin_for_hmod/
https://bukkit.org/threads/req-the-herobrine-plugin.19251/
https://github.com/darkdiplomat/iLoveYou
https://archive.org/details/iloveyou_202211
https://www.youtube.com/watch?v=BeM7O25lcr8
https://www.youtube.com/watch?v=mGR4bAlt-Xo
https://www.youtube.com/watch?v=syTY4ctONGY
https://www.youtube.com/watch?v=bjFaRSSgF_o
https://www.youtube.com/watch?v=DYC-n00pPKI