Haven't the third wheel legendaries always been dramatically different from the initial two?
Also that picture is obviously fake; it's just some really bad fanart.
Yes, they have. Kyogre and Groudon --> Rayquaza. A whale, a giant land monster-thing, and a sky dragon. Palkia and Dialga --> Giratina. Weird pink dinosaur, a large steel horse, and a centipede-like lizard (if I have to explain the centipede part one more fucking time I'll pull my hair out).
Also, the picture is obviously not real, but it doesn't say it is real. It's a drawing of what someone saw.