Since you asked, here's a distinction the internet provides: https://english.stackexchange.com/questions/30507/differences-between-sleeper-and-dark-horse
Here's a part of it:
dark horse |ˈdɑrk ˈˈhɔrs| noun 1 a person about whom little is known, esp. someone whose abilities and potential for success are concealed : [as adj.] a dark-horse candidate. 2 a competitor or candidate who has little chance of winning, or who wins against expectations : a preseason dark horse as the nation's top collegiate football team.
sleeper |ˈslipər| noun 3 a movie, book, play, etc., that achieves sudden unexpected success after initially attracting little attention, typically one that proves popular without much promotion or expenditure. • an antique whose true value goes unrecognized for some time.
Actually, Black Beauty is a "dark horse," while a lazy, old nag is a "sleeper."