Space Giraffes are the largest mobs, and appear to be 5 blocks tall and slightly wider than 1 block, though they can walk in a 4 tall x 1 wide tunnel. They are glow-in-the-dark white with stripes and spots in 2 shades of light gray. Their 4 legs are roughly 2 blocks tall with the front legs slightly longer than the rear. Their slanting bodies are 1W x 0.75T x 1.5L rectangular prisms. They do not have a clearly defined head, but their long thin necks have 2 small square dark gray eyes on the front near the top.
Space Giraffe are often found in herds. They can jump, but only 1 block high. They are slow moving and easily killed, but do not drop any items upon dying. Herds seem to disband fairly quickly after initial spawn.
Currently (game version 1.8.0), they do not appear to serve any purpose other than visually reinforcing that fact that you are indeed on an alien moon. Perhaps collecting them as pets, or to experiment on, would be an interesting challenge. See Mob Wrangling to see how to capture one.
Very occasionally a Space Giraffe will glitch, or in reality, it is two or more glitching together. They seem to meld into one entity until one moves away from the other. This can result in unusual visual phenomena like three-eyed Space Giraffes.This is easiest done if you capture two or more in a box. Also, you can raise them up vertically by Traping them in a hollow 3-3 tower, placing a block at their feet and waiting for them to jump. Repeating this, you can raise them as high as you want. This is much faster than waiting for them to jump onto a block as they turn somewhat often when they cannot move.