Choo-Choo Charlie

Choo-Choo Can't Smell too Good. Choo Choo Charlie has disadvantage on Wisdom (Perception) checks that rely on smell.

Spider Legs. Charlie walks around on spider legs, not tracks. This lets him climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Always Chugging Along. Choo Choo Charlie is always in motion, therefore he cannot be restrained, paralyzed, petrified, frightened or incapacitated. He cannot be harmed by poison or charmed. Charlie's metal hide grants him resistance to bludgeoning attacks.

Pragmatic Predator. Charlie often attempts to hide and ambush players during battle. If Charlie is bloodied or takes a critical hit before doing so, he leaves a trail of blood. This gives players advantage on any Wisdom checks made to detect his location.

Kill it with Fire. If Charlie is set on fire, it immediately moves 5 feet in a random direction his movement speed drops by half for 2 turns. Charlie must be burning due to magical or non-magical means (such as Green Flame Blade or Alchemist's Fire). 

Multiattack. Choo Choo Charlie makes two attacks with its legs. It may also use an attack to ram into people or objects.

Spider Legs. Melee Weapon Attack: +4 to hit, reach 15 ft., one creature. Hit: 8 (1d6 + 4). Target must succeed on a DC 12 Constitution saving throw or be poisoned for 1 minute. Until this poison ends, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the poison on itself on a success.

Derailing Ram. If Choo Choo Charlie travels 10 ft. or more to attack a target, it is considered a ramming attack. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 4). Target must succeed on an DC 10 Athletics or Acrobatics savings throw or be pushed 5 ft. in the opposite direction of Charlie's attack. 

Hide. Charlie will attempt to hide if he loses one-quarter of his hit points or is hit with a critical attack. He can use his eldritch powers to burrow into the ground or jump through the ceiling if he is indoors. Doing so allows him to take the Hide or Disengage actions as bonus actions on his turn. Charlie's ability to hide is limited by the surface he is currently traveling on. If Charlie is on the ceiling, he will burrow into a space on the ceiling. If Charlie is on the ground, it will burrow into the ground. 

Pounce. If Charlie's location is not revealed before his next attack, it has advantage on its first attack against his first target after reappearing.  

Devouring Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 4) piercing damage. Choo Choo Charlie may only use this attack as a reaction on a target within 5ft when when the target is bloodied, paralyzed, or otherwise immobilized.