I have also seen Silhouettes of weedle and seedot, sableye. I already have these [duplicate]

Solution 1:

TL;DR: It's probably a bug.

Pokemon GO just added Turtwig and several other Pokemon to the list of available shadow Pokemon. People have previously determined through data mining that when a shadow Pokemon is added, the internal name for the normal Pokemon changes. In this case, the internal name TURTWIG would be replaced with TURTWIG_NORMAL when TURTWIG_SHADOW and TURTWIG_PURIFIED are added. It is likely that the same was done here and that a coding mistake made the new normal Turtwig seem to be a different Pokemon from the original Turtwig.

This renaming previously caused a different bug that prevented normal forms of Kanto Pokemon that also had shadow forms from being transferred to the Pokemon Let's Go Switch games.

People have reported experiencing the same thing in this Reddit thread, and have said that sometimes when they try to catch that Turtwig the game crashes, and if they do catch it the Grotle image in the evolve button is also a silhouette even if they have previously evolved another Turtwig.