Quote from JoshBrodieNZ >>

What's stopping someone from using polymorph to become one?

 Nothing.

I wouldn't recommend polymorphing into a Giant Ape to grapple an Ancient Dragon. To quote from the spell:

The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.

So if Rage is a game statistic, then you don't get to keep it. Crawford agrees: https://www.sageadvice.eu/2018/01/28/why-does-polymorph-stops-an-active-rage-on-a-barbarian-but-doesnt-remove-arcane-ward/

So let's say a giant ape get's into a grapple with an ancient dragon; an ancient white dragon because it's probably the wimpiest dragon you can face off against and still be called a dragon. The dragon has a +8 Strength bonus and no proficiency in Athletics. The giant ape has a +9 Strength (athletics) bonus so the margin for making the grapple check is only 2.5%. Ouch.

New, let's consider the dragon's attacks. If the dragon uses multiattack, it makes the following attacks:

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack:+14 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8)piercing damage plus 9 (2d8) cold damage.

Claw. Melee Weapon Attack:+14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8)slashing damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

On the dragon's turn, it uses Frightful Presence with a 70% change of the ape just giving up and going home. Next, the ape only has an AC of 12 so the dragon is going to land all of its attacks barring any crit fails. Total damage per round from the dragon is 58. Three rounds later, the polymorphed character is back in his original form with 17 fewer hit points then when they started.

The giant ape is really, really good for dealing with small squishies. Not so hot for going WWE on dragons.

It looks like this was fixed.

How tall is monks asking for little brother

Mmm monke

As a wise man once said "Ook"

Quote from theawesomeguy >>

you can have moon circle and turn into it at lvl 14 as a druid using wild shape, and I use polymorph to turn into it

 

 

 Wildshape for moon druid is Level/3 = CR beast.  In other words, you need to be a level 21 Moon Druid to wild shape into a Giant Ape

Going up against an Ancient Green Dragon right now. My fighter got Polymorphed into one. LET'S GOOOO!!!!

"Multiattack. The ape makes two fist attacks" - can giant ape get 2 rock attacks or only 1 rock attack? 

return to monke

This game really needs rules for throwing another creature. 

Armor of Agathys as a Mark of Warding dwarf abjurer, then giant ice ape time. Go ahead, hit me -- you'll take 20 cold damage each time, and then I get to reply with my furious ape-fists!

Big monke

Like kneading dough with lethal intent.

Look man you're obviously coming from 1st  or 2nd edition where homebrew is heresy and creativity is a sin and if you play 5e and you think homebrew is wrong you're missing the whole point of the game

Who is "obviously coming from a . . . creativity is a sin" perspective? 

If you are referring to me, then I'm afraid you have misunderstood both the intent and the tone of my post. ;-)

I'm the guy that suggested giving the ape a higher AC, four arms, and a jet pack if you want. Give it hyper intelligence and telepathy like Gorilla Grodd. Or give it fey blood and the ability to disguise itself as a tree or cast Blur. Do whatever you want in your own game, as long as its fun for the people at your table. 

However, the question I was answering was directly about understanding the RAW, so I explained the rule in question. Rogues and Wizards and Giant Apes don't have proficiency with shields even though they all have hands. 

And for what it's worth, I have been playing since 1981 and have never been bothered by creative and interesting gameplay. Of course, I was poor then. Since I never bought any of the books or modules, creativity was critical. ;-)

yes. very yes

Think Hulk.....  then back to super genious when spell ends.

HULK SMASH!!!!!

Circle of the Moon?

Homebrew this into a giant bat-like creature, give it spider climb, blind sight and a deafening/stunning scream.