Ilsty and ? is an effect only in Pokémon Gold and Silver that happens when trying to view the glitch Unown hex:1C (dec:28). They are not obtainable Pokémon.
It can be seen with ????? party overloading when withdrawing 5 Pokémon from a full party (putting them at the top of the party each time) when having a fifth party Pokémon with a Defense stat of 28 modulo 256 (see the glitch Unown article for steps and more information).
This will put glitch Unown hex:1C (dec:28) as the first Unown in Unown Mode of the Pokédex, but it has the bad side effect of corrupting the player's party Pokémon, the withdrawn PC Pokémon and the seen and own addresses for the Pokédex.
With a cheating device, they can be seen with the GameShark code 011C24DC. This cheat also changes the first Unown in Unown Mode of the Pokédex.
The game will switch to the evolution screen, display a glitchy area of text, then act like a Pokémon named "ILSTY I PkPkPkHpPOKé ÄMn POKé" is evolving into a Forretress, even though the picture does not change to Forretress's sprite. Afterwards, a Graveler named "?" will evolve into Golem with the correct picture. The game will reset in a Glitch Dimension afterwards. No other variation of the effect has been documented.
The glitchy area of text.
"Congratulations, your ILSTY I PkPkPkHpPOKé ÄMn POKé"
"evolved into FORRETRESS." (note the picture is not Forretress' sprite)
"What? ? is evolving!" (? has Graveler's picture)
"Congratulations, your ?"