It's like when you get assigned a college roommate, and meet her for the first time.

As far as their physiology once they are fused together goes, the spell was designed to retain a lot of the dragon's endurance, resilience, and strength, while overall the body plan favors the unicorn side to maximize travel. Outside of the manes and tails there isn't any more hair, and they're wrapped completely in dragon scales. Some of the pony features vary from union to union like the muzzles may change to match their dragon host a little closer, or which host the colors of their new body will adopt. Sometimes they're completely the dragon's, sometimes the unicorn's, and a few even have a blending of some of the pallets and patterns making something new to both of them. However, one constant is both members always share whatever eyecolor they seem to get.