Each of the four evangelists – writers of the New Testament gospels in the Bible – had their own symbol to identify them in Christian art: Matthew, angel; Mark, lion; Luke, bull; John, eagle.
Local examples (click to enlarge)
The Evangelists with their symbols, depicted on the font in the Cathedral nave (from left): Matthew, Mark, Luke and John
Wall painting, Christ with the evangelists, Eastbridge Hospital (q.v.)
Corona south window - four Evangelists surround Christ