A Caimo is a very clever mob, and actually very difficult to spot. At first, when they spawn they look like Steve but after they are seen they use their unique ability to disguise themselves as nearly ANY block. They can also turn into blocks sometimes when they disguise themselves against a wall they will turn into 2 blocks stacked on top of each other, Some blocks they cannot turn into, such as chests, water, lava, doors, pistons, Redstone wires and Redstone repeaters etc.
Spotting a Caimo
Sometimes when a player looks at a Caimo you may see some particles of the block they have just destroyed to disguise themselves as, and if the player hits them they will react and attack the player.