Invertebrate sentience
When people hear animal, they usually think of vertebrates, but invertebrates such as insects are also included among animals, and it seems that quite a large number of species possess sentience.
Empirical research on invertebrate sentience
There is a study1 comparing features related to conscious experience in a total of 18 kinds of living beings, including plants, some vertebrates, and various invertebrates. It investigated a total of 53 traits across eight categories: responses to harmful stimuli, motivation-based trade-offs, learning-related indicators, cognitive sophistication, responses to drugs, navigational skill, behaviors related to emotional states, and anatomical/evolutionary features. (Reference: Sentience table)
According to that study, when plants, chickens, and cows were compared, plants showed large differences from chickens on nearly all 53 traits, whereas chickens and cows showed more similarity than difference.
The following is the result comparing only cows, chickens, and plants.
If we extract only the traits related to pleasure and pain, they are as follows.