Arceus does not have 1,000 arms. It's only a legend.
Arceus is an equine being similar to a Qilin or Centaur. Its body color is white with a gray, vertically-striated underside, the pattern of which has similar recurrences on the underside of Arceus's mane, tail, and face, and Arceus's four pointed feet are tipped with gold hooves. Its mane is quite long, jutting away from its head, and its face is gray, with green eyes and red pupils, and a green circular pattern below its eyes. Arceus also has a streak of gold coloration on its head, and ears that point upward. Its neck is fairly long, with two pairs of extrusions to the sides, and a flap-like feature on the neck's underside that is colored white like much of the body. It also has a golden cross-like wheel attached to its body by its round abdomen, which changes color along with its eyes and hooves based on Arceus's current type. The wheel also has four jewels attached to it. The pattern of striated gray of Arceus's underbelly resumes past Arceus's waist, the underside of Arceus's limbs also being gray in coloration, and with extrusions at the tops of the legs. Arceus's tail is fairly like its mane in shape and coloration. According to legend, it has 1000 arms.
The Pokedex can actually be very misleading at times, like on Weezing's page it explains that triplets have been found, though not legitimately obtainable; untrue. The situation is similar with Arceus. Nice catch though ;)