Mickey Mouse: Disney Animated Shorts

The Chain Gang (1x21)

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The first appearance of Pluto, surprisingly enough as one of the hounds who chases Mickey after his escape. Mickey's a prisoner on jailer Pegleg Pete's rock pile. In a riot, he escapes, but when he's pursued by the jail guards' hounds, he jumps on a pair of motley horses who finally return him to jail the hard way.