The emperor Nero in "Roman Legion-Hare" is a caricature of Charles Laughton's portrayal of King Henry VIII in "Young Bess" (1953).
"E Pluribus Uranium" is a play on the United States' motto, "E Pluribus Unum" ("From many, one" referring to the single nation arising out of the union of many states). Bugs Bunny's version would translate, roughly, to "From many, uranium."
The caption at the beginning says "ROME 54 A.D." A.D. 54 was the year Nero became emperor of Rome -- 1901 years before 1955, when the cartoon was made and released.
The film anachronistically portrays Emperor Nero (37 - 68, reigned 54-68) taking a seat at the Colosseum of Rome. The proper name of the Colosseum was the Flavian Amphitheater (Latin: Amphitheatrum Flavium). The construction of the amphitheater started in 72, by orders of Emperor Vespasian (9-79, reigned 69-79), the founder of the Flavian dynasty. The construction was competed in 80, a year's after Vespasian's death. The emperor during the amphitheater's inauguration was emperor Titus (39-81, reigned 79-81), Vespasion's oldest son.