Croak

at lvl 4 druid you get to eat people.....

If you give this, or a regular Frog Hypnotic Pattern you literally have THE HYPNO TOAD

I think I'm missing something or it's an error.  In Hoard of the Dragon Queen, Ch 6, pg 50 under the Huts section,  it mentions Giant Frogs.  It then says, "Eight giant frogs hop randomly among the huts or sit silently in the pools of standing water. These creatures were raised from tadpoles by the bullywugs and don’t bother them, but they attack anyone else who carelessly wanders within reach of their 15-foot-long tongues."  

I don't see anything in the description above about Giant Frog's tongues and having a 15 foot reach. 

A beast master Ranger. The possibilities are infinite.

Quote from Daraise >>

If you give this, or a regular Frog Hypnotic Pattern you literally have THE HYPNO TOAD

 That would be a Hypno FROG, not toad. But there are toad and giant toad...

If you’re a Druid and you shapeshift into this, swallow a creature, and then drop the form.... what happens to the swallowed creature?

If I had to guess, the swallowed creature would just appear beside you, being prone as if they exited normally. After all, shape shift only affects you, not anybody else. Although, it would be pretty funny to see a demi-lich be killed by consumption.

So "The frog makes one bite attack against a Small or smaller target it is grappling"

As it has already had a turn to Bite (and Grapple) does it have to wait until its next turn to do the Swallow, or is it implied that if it makes the grapple it can swallow?

Konosuba.

Pain. Lots of pain.

Swallow is on the following turn, giving the creature at least one chance to break the grapple. That is how i ruled at least.

Grapple has the following statement:

"...If you're able to make multiple attacks with the Attack action, this attack replaces one of them."

Beast masters have the following feature:

"Starting at 11th level, when you command your beast companion to take the Attack action, the beast can make two attacks, or it can take the Multiattack action if it has that action."

I see no reason a giant frog companion couldn't attack a small or smaller creature with a DC 11 strength/dex check, and eat it too.

(Assumed strength/dex check since grapling works this way)

If it only has 1 attack then you'd have to wait a turn.

Hanzalf, I'm running HDQ right now and just encountered the same issue. I'm assuming that the tongue has to wrap around a target at least once, possibly twice, to achieve the grapple. That would require 5 or more feet and could account for the discrepancy.

Or it could be a custom feature for HDQ, like how EGW's Frozen Sick has them as Ice Frogs (immune to Cold damage).

I'd rule it as a restrained-in-place, then next turn, YOINK! and chomp.

not people...Gnomes. *burp*

I'd say it has to wait one turn since none of these attacks are marked as bonus action or similar.

So, if your DM let's you choose what you conjure with conjure animals, 8 giant frogs would be quite stupidly powerful wouldn't it? They grapple on each attack, and if they swallow an enemy it would pretty much keep them out of the fight for the duration of conjure animals, while taking damage every turn, because there doesn't seem to be any way to get out of the frog other than its death. Imagine the look on the bbeg's face when the druid waves their hand and all their henchmen are swallowed by frogs.

Quote from Montell >>

So, if your DM let's you choose what you conjure with conjure animals, 8 giant frogs would be quite stupidly powerful wouldn't it? They grapple on each attack, and if they swallow an enemy it would pretty much keep them out of the fight for the duration of conjure animals, while taking damage every turn, because there doesn't seem to be any way to get out of the frog other than its death. Imagine the look on the bbeg's face when the druid waves their hand and all their henchmen are swallowed by frogs.

Other creatures with the Swallow action, such as the remorhaz and the purple worm, allow swallowed creatures a chance to escape if they deal enough damage to them in one turn from the inside, and even if they don't manage to meet the damage requirement the existence of this trait implies they can just poke the thing to death from inside anyway. So I think it's safe to say creatures swallowed by the giant frog can do the same.

As for why this feature doesn't appear in the statblock, I imagine the damage required for the frog to regurgitate them would be either insignificant or high enough that it would just die anyway (eg.: for both creatures I mentioned it's 30 damage).

EDIT: Also, the giant frog can only swallow Small and smaller creatures, so unless your BBEG's army is entirely composed of halflings they shouldn't be that problematic. Giant toads can swallow Medium creatures, but you can only summon two of them with 3rd-level Conjure Animals so they shouldn't be too bad either unless the druid uses a higher-level slot.

I should make a (tongue variant) of these guys. Also, weirdly enough, this roughly dwarf sized creature can grapple and RESTRAIN a creature, regardless of size or Athletics/Acrobatics. A group (or even one) of these guys can be a serious pain in the ass to even a giant, forcing them to waste actions, divert attacks, or live with being immobile and restrained. 
When it comes to monster statblocks, it's left up to DMs to make sure weirdness doesn't result when Enlarge, polymorph, wild shape, and conjure animals bring up things that wouldn't often happen.