Himmelhund is pronounced as in German, roughly "him-ul-hunt".
They resemble grey or white dogs, with gold markings or patterns on the fur and gold eyes (which resemble solid matte gold marbles from most angles). Heavenhounds appear different sizes at different times; while hunting they will be foxhound size   unless the prey is bigger. When seen in smaller groups they can be (or seem) much larger.
Himmelhunde are found in packs, never singly and rarely in smaller groups. Individual heavenhounds do not bind to angels as hellhounds to demons; in fact, himmelhunde do not generally consider themselves individuals at all. It's especially noticeable when a pack is on the hunt, but himmelhunde seem to share each other's thoughts and experiences all the time.
Heavenhounds' baying causes immediate and severe paralysing terror in anything but an angel, although they are able to manipulate fear by other means too. They do not seem to speak. It's unknown whether they ever communicate telepathically with other creatures. Nobody is even sure whether their hive mind works similarly to known forms of telepathy.
Like hellhounds, himmelhunde frequently set out to hunt. However, while Hadean hunts are raucous and tend to the chaotic (demons not being terribly good at cooperation), himmelhunde on the hunt are single-minded and barely need the intervention of a supervising angel.