In Creatures 1 all food was simply called "food" (Lemons, Honey, Carrots, and Cheese were the basics). Norns could eat food and plants. However, there was no verb "eat", so creatures had to "push food/herb/weed". Chris Double created some Eater Norns and modified a COB in order to get norns that... well, he can probably explain it better.
In Creatures 2 the word "eat" was created, and some new foods and plants were added, including a new food group: 'fruit'. Thirst was also added (though it was removed again in C3), but there was no verb 'drink' so Norns had to 'eat drink' when thirsty.
In Creatures 3 the things a creature could eat were expanded, and food began to decrease different types of hunger (Protein, Starch, Fat).