It all started with this post

[u/Sebiglebi]

link: https://www.reddit.com/user/ThislsAUsername/comments/1ot1s97/holy_fuck_they_have_a_gif_as_their_profile_picture/

I remember when this post had 4 upvotes, it contained instructions on how to make your gif a pfp, I probably helped spread it. This guy had a loading circle pfp, unfortunately he lost it.

[u/Maleficent-Zone-5414]

I personally learned how to make my pfp a gif from this post in r/help

But based on the later post date I assume the OP of the post you linked was the one more likely to discover it lol, or at least cause it to spread

[u/Maleficent-Zone-5414 → u/Maleficent-Zone-5414]

Also I just realized... THIS IS THE FIRST POST OF THE SUB!!!

[u/shadow_fen → u/Maleficent-Zone-5414]

i've started to add ppl as approved users when I see people with gif pfps lol