Is there any difference between an ursine and an ursid?

Solution 1:

They both relate to bears and the bear family. Each definition is thus

Ursid

A. adj. Of, pertaining, or belonging to the family Ursidae. B. sb. A mammal of this family.

Ursidae from Merriam Webster

a family of large powerful plantigrade carnivores including the bears and extinct related forms

Ursine from my OED2 again

Of or pertaining to, characteristic of, due to, a bear or bears.

So Ursine is an adjective while Ursid is both a noun and adjective. Ursid leans slightly to the classification whereas ursine leans to a bear's characteristics, e.g., size, power.

If I called a man who is big an ursine man/monstrosity, then I would be talking about his resemblance to a bear/having a characteristic of a bear. If I called him, figuratively an ursid, the same meaning is imparted. But if I said he had an ursid appetite, that wouldn't be very appropriate use because the appetite has nothing to with the family Ursidae

Ultimately, in this context, ursine and ursid are basically interchangeable because a bear does pertain to its characteristics and the family Ursidae.