From 1964, here's a "Compact Box" of Post Crispy Critters with Linus the Lionhearted. The folks at Post Cereals decided to reduce the sizes of their boxes of cereals while maintaining their former weight. Customers, though, were not impressed, thinking that they were being cheated out of an amount of cereal. By 1965, General Foods, then the parent company of Post Cereals, abandoned the Compact Box project.
From the late 1950s, here's a commercial for a short-lived Post Cereals product, Heart Of Oats, with an early version of Linus the Lionhearted (voiced by Sheldon Leonard). The character would soon be redesigned and would become the mascot for Post Crispy Critters (as well as the star of his own TV cartoon series, Linus the Lionhearted).
From 1964, here's Linus the Lionhearted plugging Post Crispy Critters.
The lovable King of the Beasts---voiced by veteran character actor-turned-producer Sheldon Leonard, was the star of a television cartoon show entitled Linus The Lionhearted (CBS 1964-66; reruns on ABC 1967-69) which starred characters that were connected to respective Post Cereal products (Sugar Bear for Post Sugar Crisp for example).