I’ve seen a popular blog about Delta Pidgey-Pidgeot and that was the same ones as the ones that were added, SAME ARTWORK
You must be referring to this post.
Yes, it was made by a fan. In fact, most of the delta lineup (I want to say around 60-70%) of the deltas were fan submissions. They were submitted in holon university much like other fan submissions that made the cut. All fan made creations that were used had the consent of the artists who them.
You can tell which ones were made by fans by looking at their first stage’s dex entries.
A good portions of deltas were fan submitted ones that we, with permissions from the original owner, aded into the projects. Even their names are featured in the credits, and the pokemon’s pokedex entries specifically state who created them originally.