Summary: The meme is a recolour of Big Macintosh that has quickly spread through the Brony community on Instagram.

Origin:
On Jan 6 2014, an instagrammer under the username insta_flashsentryy uploaded a lazily recoloured scene of Big Macintosh from the MLP:FiM show, to make it look like the character had a turquoise mane and a medium purple coat. The user used the word "Roberto" as the caption, to state the name of the recolour.

The Spread of the meme:
On Jan 15 2014, another instagrammer under the username worstocs reposted a screencap of the recolour, and also used Roberto as a caption to troll the originial creator. Many followers of the instagram page started to notice the recolour and repost just like worstocs.
Many more "bad oc" accounts followed on reposting the lazy recolour.
On Jan 18 2014, horribleocs posted a picture of the recolour with a purple frame around it. Inside the picture was a tag by the name "#inrobertowetrust".
Overnight, the tag started to feature many more Big Macintosh scenes recoloured to be Roberto, and in only 1 day, the tag had already hit 121 posts featuring the tag.

Many more phrases were used to relate to his existence (as a joke, of course), for example:
"Notice me senpai!!1!!!! <3"
Or
"Roberto is love,
Roberto is life"

Fans:
There are some accounts on instagram that are fanpages of the recolour, and many people have started to "fanart" the recolour due to its popularity. To show their love for the character, many instagram accounts change their profile picture to be Roberto, or they join the "Roberto Empire" which is a huge fan-community of artists, fans and much more.

The original creator of Roberto is currently inactive, but if they were to come back soon, they'd be in for a big shock.
Possibly a good one.
POSSIBLY.