Looks like all we have left is Skills, a Challenge Rating, and a Description.
I spoke earlier about a racial bonus to Listen & Spot, how about borrowing a whale's "*A whale has a +4 racial bonus on Spot and Listen checks. These bonuses are lost if its blindsight is negated."
Although, considering its lateral lines are pressure-sensitive I suspect their Blindsight would be hard to negate!
Put the odd point in Move Silently, I guess, to make it:
Minnow Form Skills: Hide +23, Listen +11*, Move Silently +8, Spot +11*, Swim +15 Monster Form Skills: Hide -2, Listen +11*, Move Silently +3, Spot +11*, Swim +13
*A death minnow has a +4 racial bonus on Spot and Listen checks. These bonuses are lost if its blindsight is negated.
Challenge Rating 4? I'd eyeball it at CR 3 because of its swallow whole, but its size-shifting trick should shift it up a CR.
There's plenty for a description, at least by the paltry standards of 3rd edition.
A bright orange fish the size of a human's little finger.
A death minnow is an artificial lifeform, created by unknown undersea spellcasters to guard their wealth and secrets. They are only ever found guarding a site or object, there are no "wild" death minnows. They do not (or can not) breed, but are created magically. It is uncertain whether they have male and female sexes.
Deaths minnows will attack any dangerous intruder who enters an area they're set to guard, whether animal predator or adventurers. They can go for extraordinary lengths of time between meals, since they can have the appetite of a minnow and the stomach capacity of a giant angler fish.
This magical beast has the supernatural ability to shrink to the size of a minnow, together with anything it has swallowed. Its natural size is a monstrous fish resembling a giant grouper, with the ugly head and protruding lips of an angler fish; this monster form is typically 8 feet long and weighs about 500 pounds. In minnow form it averages 2 inches long and weighs a fraction of an ounce.
Yes, I was thinking more of the SRD monsters. There are some 3E beasties that have well developed flavour.
They're certainly no worse than many, if not most, 1st edition monsters. It's 2E when the flavour text really spread.
And even then, there are a fair few monsters who's flavour text has the whiff of being extremely padded out to fill up the 1-page Monstrous Compendium format, without actually saying much of interest to the DM or players.