How large must a creature be for the “Climb Onto a Bigger Creature” action?

Question

From the DMG:

Climb Onto a Bigger Creature

If one creature wants to jump onto another creature, it can do so by grappling. A Small or Medium creature has little chance of making a successful grapple against a Huge or Gargantuan creature, however, unless magic has granted the grappler supernatural might.

As an alternative, a suitably large opponent can be treated as terrain for the purpose of jumping onto its back or clinging to a limb.

The first paragraph of the action references the inability of normal-sized PCs from grappling Huge or Gargantuan creatures, but it doesn't clearly state whether the action can only be taken for those creatures, or if it can be taken on simply larger creatures. Throughout the rest of the section, it refers only to the difference between larger creatures and smaller creatures.

This begs the question, can a Medium-sized PC climb onto a Large opponent (think Legolas climbing onto the cave troll in Moria)?

Answer 1

By rules as written, you can only grapple creatures one size larger than yourself (PHB p. 195), so a medium creature can jump on a large and a small onto a medium.

Note that the second paragraph is an alternative, not a variant, so it too is in play. If the creature is larger than you can grapple and is suitably large, whatever the DM decides that is, you can simply use your movement to walk or climb it. The DM would need to decide which and if the creature was difficult terrain or not and if any checks are needed; presumably Dexterity (Athletics). Assuming the creature didn't want you there it could try to throw/pull/buck you off, resolve this as an opposed check.

Personally, I think it would be awesome if the players jumped and climbed up the dragon to try and sever its spine. They would too; right up to the point where I say "The dragon drops prone and rolls over you. Time for some saving throws."

Answer 2

The rule as written does not in any way restrict the size of the creature that is to be climbed on. So, it would seem that any creature that is:

1. Larger than you by at least one size category (as indicated by the title "Climb onto a BIGGER Creature")
2. Of a size that you can grapple

you may use grapple to traverse the creature.

OR

if the creature is:

3. "Sufficiently large" (DM discretion) and
4. You cannot grapple it because of its size

then your DM may rule that you may use terrain rules to traverse the monster.

Seeing as this is a general rule with no contradicting specific rule, I think it is very fair to just take it at face value and say that any creature meeting the above two requirements are able to be climbed.

Rationale

If one creature wants to jump onto another creature, it can do so by grappling.

(Emphasis mine)

This is unambiguous. To climb onto the back of a larger creature you need to use a grapple. Thus, this must follow the grappling rules.

A Small or Medium creature has little chance of making a successful grapple against a Huge or Gargantuan creature, however, unless magic has granted the grappler supernatural might.

Again, multiple explicit call-outs to grappling which is clearly what we are mechanically doing here. Using a grapple to climb onto a larger creature's back. Additionally, it says that you can't grapple a creature more than two sizes bigger than you (restating the grappling rules).

As an alternative, a suitably large opponent can be treated as terrain for the purpose of jumping onto its back or clinging to a limb.

This phrase is referring to the previous paragraph. As in, "As an alternative to using grapple to climb a larger creature...". It is saying that if the creature is sufficiently large (left up to the DM it seems to decide what qualifies) then, instead of grappling (because you literally can't grapple them), you can treat the very big creature as terrain.

TL;DR

To climb a creature, use a grapple if it is a size larger than you that you can grapple. If its bigger than you can grapple, your DM must decide if its sufficiently large enough for you to treat it as terrain.

Answer 3

I'd say that its possible, but be wary - it provides advantages over regular grappling.

Just use common sense and you should be fine.