dtgreene: The Dragon Quest series has Slimes, which look like purple Hershey's kisses with smiley faces.
Sarisio: Since then Slimes have become iconic for most RPGs. So they aren't truly flagship. DQ has a lot of recurring monsters - Lipsies, Gracos (recurring boss), etc.
Final Fantasy: Malboro, Tonberry. Wanted to write "Cactuar", but when I played EverQuest II , it had tons of Cactuar enemies.
Arc the Lad: Bonze.
Breath of Fire: Beak.
Disgaea: Maijin.
Phantasy Star: Fiend (name in PS II), Dark Force is flagship boss.
Some series don't have flagship monsters. Can't remember flagship monsters in Might and Magic series.
I really don't see Malboro and Tonberry as iconic to Final Fantasy when neither monster appeared in that game. (Malboro first appeared in FF2 and was absent in FF3 and FF5, Tonberry first appeared in FF5, and Cactuar in FF6.) I see goblins as being more iconic, being one of the first enemies fought in the first 5 games in the series (and necessarily FF3 and FF5).
Many RPGs have monsters called Slimes, but how many have Metal Slimes?
Consider also the following strange encounters of them:
In any Dragon Quest from 1 to 7 (8 and 9 don't let enemies flee during the surprise round):
A Metal Slime draws near.
The enemy attacked before you were ready!
The Metal Slime runs away.
Or the following, most likely happening in 6 or 7 (because of the way the class system works):
A Metal Slime draws near.
[The command "RUN" is chosen.]
The party tries to run away!
But the Metal Slime blocked the escape!
The Metal Slime runs away!