Yes, it grates on me, too.
In the case of a ten-pound pony, though, since it’s an adjective (a compound adjective?) that’s different.
I had to think about something similar when doing an OU course on the 19th century novel. One of our student email group was asking (for her essay) when ‘nineteenth century’ should be hyphenated, and when not.
I’d never consciously thought about it before, but when it was adjective plus noun, as in, ‘during the nineteenth century’ obv. no hyphen, but when they were coupled as an adjective, as in, ‘nineteenth-century literature’, hyphen needed.