This creature is a giant octopus that has evolved to live and thrive on land. It can alter its coloration to appear as at rock formation and it tends to lurk in crevices and fissures, attacking smaller creatures that wander near. It has a walking speed of 20 feel and a climbing speed of 10 feet, loses its Hold Breath feature, and replaces its Underwater Camouflage feature with the following feature:
Camouflage. The octopus has advantage on Dexterity (Stealth) checks.
Here's the XML for it, copied from the Giant Octopus XML fragment, and edited as noted above. I have also removed the Water Breathing feature. The Ink Cloud feature has also been removed because it's not much use on land...the description doesn't state whether the Rocktopus is amphibious or if it has lost its ability to live underwater - but the latter is implied because it has lost its 60' swimming speed.
<monster>
<name>Giant Rocktopus</name>
<size>L</size>
<type>beast, Out of the Abyss</type>
<alignment>unaligned</alignment>
<ac>11</ac>
<hp>52 (8d10+8)</hp>
<speed>20 ft., climb 10 ft.</speed>
<str>17</str>
<dex>13</dex>
<con>13</con>
<int>4</int>
<wis>10</wis>
<cha>4</cha>
<skill>Perception +4, Stealth +5</skill>
<senses>darkvision 60 ft.</senses>
<passive>14</passive>
<cr>1</cr>
<trait>
<name>Camouflage</name>
<text>The octopus has advantage on Dexterity (Stealth) checks.</text>
</trait>
<action>
<name>Tentacles</name>
<text>Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the octopus can't use its tentacles on another target.</text>
<attack>Tentacles|5|2d6+3</attack>
</action>
</monster>
From p. 28 of Out of the Abyss:
Here's the XML for it, copied from the Giant Octopus XML fragment, and edited as noted above. I have also removed the Water Breathing feature. The Ink Cloud feature has also been removed because it's not much use on land...the description doesn't state whether the Rocktopus is amphibious or if it has lost its ability to live underwater - but the latter is implied because it has lost its 60' swimming speed.