While not exactly rare, giant toads aren't particularly common either (at least in my games). It only shows up randomly on the wilderness tables and there only at rivers. I think this is because it is a relatively low level monster that doesn't make its appearance until the Expert rules. The river table appearance is somewhat limiting when one considers that real-life toads are often found well away from large bodies of water. Nevertheless, Mr. Toad is here and we shall investigate him before he drives off.
Toad, Giant (from Cook)
MV: 90' (30')
# APP: 1d4 (1d4)
GTs are described as 150-250 pounds ("the size of a very large dog") and capable of changing the skin color. This chameleon effect increases their surprise chance slightly (1-3). Their hit dice and AC are no great shakes and they are hardly the bravest creatures when it comes to morale. Not to mention no real loot to speak of.
The two things that make the giant toad interesting are its ranged tongue attack (15') and the chance of swallowing a target whole. Only "small" (the text implies dwarf-sized or smaller) targets can be swallowed, and only then on a natural 20 attack roll. A swallowed target takes 1d6 damage per round while in the amphibian's gullet.
I think the reason this didn't get included in Moldvay was A) space limitations and/or B) the creature's wilderness aspect. It is after all just a big animal. The Expert rules opened up the wider (and wilder) world beyond the dungeon to the players & DM, so it seems -aheh- natural that some less powerful yet monstrous wildlife was introduced.
As far as running an adventure with these creatures, I would happily place a few near a river or swamp and have them squat silently like a stone as the party travel through. Then THWIP! A sudden lashing out of a tongue at the halfling or dwarf PC to drag them in for a bite and if lucky, GULP! The emphasis (for me) would be to underscore the fantastic elements of a BX world rather than as a major encounter or challenge.