My confusion comes from this part of the polymorph spell:
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech.
So, for example, a Giant Ape is obviously naturally able to grapple – does the polymorph restrict it, though? In other words, does the usage of "other action" here mean "other than the natural abilities of the creature", or "other than those specified by the spell"?
Best Answer
Generally1, the Polymorphed creature can't grapple
My reading on the other action is other than spells. Grapple requires a free hand, thus it obviously requires hands.
This reading means polymorphed creatures can not take the Grapple action. (Yes, I'm aware Grapple is not an action - it's just a special Attack action, but I think this way is easier to understand.)
There are exceptions
Monsters don't (necessarily) follow usual Grapple rules, as stated in the MM p. 11.
This leads to some monsters being able to grapple creatures without actually requiring hands. For example, a Giant Octupus has the action
This action does not require hands, thus it is allowed. The action states that the creature is grappled, so the creature is grappled. Thus, Giant Octopus can grapple normally through the use of Tentacles.
For other examples, T-rex and Giant Toad also have actions that grapple using Bite, which is more far away from "hands" than the tentacles mentioned above.
Crawford has also spoken on these matters:
Note that this doesn't follow the usual "These are the official rules" style of writing from Crawford - it just says "I would allow it", making unclear if that's how it actually works. This is an old tweet, though, and that might be the reason for it. Also there is a lack of "allow only if", but I guess that's implicit easily.
Possible house rules
Personally, I don't see any (balance/mechanical) problems with allowing a Giant Ape to grapple normally. I would think the restriction of using hands was more for locks, doors and other things requiring Dex than actual combat actions. My reading on RAW and Crawford's tweets say this isn't the official rule, though.
1 Here, generally means that it's the rule that holds true unless stated otherwise, e.g., unless the beast has a specific action that lets it grapple without hands, as described in the following section. The fact that this is the general rule is the reason beasts are required to have the specific action to be able to grapple as we would usually imagine, for example, a Boa Constrictor doing.
2 Since the "general vs specific" discussion is constant, I'd like to emphasize that the Monster Manual itself says these attacks are special, i.e., not the general/usual rule. These are exceptional, special, specific attacks described in their statblocks that apply exclusively to the monster in that statblock, thus it can't be general in any way. Even if many monsters have these special attacks, all of them are specific to each monster, and the general rule is still applied to all of them.