The giant frogs in the Hommlett moathouse moat are iconic monsters to me. Nothing like seeing your little brother's halfling get swallowed by one his second time playing D&D to make an impression!
There were various frog monsters developed in that WotC campaign column a few years ago.
Giant Caecilia in Expert Rules/Rules Cyclopedia
Amphidragon from Dragonlance
Some poisonous toad humanoids from Greyhawk - Grung or Krung, something like that.