It's very funny to me that the stereotypical gelatinous cube is bright fucking green when the monster itself is almost perfectly transparent. Like its gimmick is that it's a monster that imitates an empty 10x10 hallway. How many people have fallen victim to gelatinous cubes because they "know" that the ooze is bright green and so don't bother to check the suspiciously clean corridor in front of them.
imagine, a non-acidic gelatinous cube. its ooze is like that oxygenated fluid mice can breathe. organisms inside it bob peacefully, harmlessly, inhaling oxygen-rich slime. the cube, in turn, absorbs respiratory byproducts that nourish it. it adjusts its atmosphere to soothe and pacify occupants and intuitively ejects unwilling ones. found in dungeons inhospitable to other life, like passages filled with poison gas, it seeks out adventurers to ferry across those areas, offering a mutually beneficial exchange of sustenance.
Level 01 Wizard, Day 01: Cast a spell to summon a familiar. Gelatinous cube, named Cubert. Immediately become friends.
Level 20 Wizard, Day 01: Cast a spell to imbue Cubert with magical power, they are now a Level 01 wizard adventurer and your apprentice. Magic-assisted hug.
On the left, Nothic! Wizards transformed by curses or weird knowledge, so whatever normal wizard height is. It also has manic energy as it act on impulses even it doesn't understand, and can learn random facts about you by staring! just note that it will be looking at you, and will just be constantly learning all about you while you're going at it. but hey, that means you can skip introductions and smalltalk, it'll figure it out at a glance!
On the right...OK hear me out. In lore they were all sentient, but MOST had that sentience taken away by their god on an angry whim. HOWEVER they sometimes get that sentience back, we've even seen one in a recent adventure. So we're talking about the sentient ones. I mean it is still just a 10x10 foot (3 by 3 meter) cube of acidic slime that engulfs you. but it can also form a tentacle thing to slap with!