Beholders just look silly
The rust monster is worse because it might eat all your magic weapons and armor. A fate worse than death according to many players.Never pick a mascot that wants to eat you.
No way how can you possibly call this silly looking? Blasphemy
I'd be down for a Umberhulk - its a much cooler bug than rust monsterHard to go against a dragon since that is in the game's name. But if it is for something more unique to D&D, it might be the Beholder. As a kid it would have either been the Rust Monster or Umber Hulk though.
I wouldn't be mad if it was the Gelatinous Cube either.
An umber hulk busting through a wall to surprise player characters would be a great dynamic image.I'd be down for a Umberhulk - its a much cooler bug than rust monster
Like on the cover of the 2E FR Stardock AdventureAn umber hulk busting through a wall to surprise player characters would be a great dynamic image.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.