One of Jesus’ disciples was Simon, who Jesus renamed Peter.
Sometimes the Bible refers to him as Simon (47 times) and other times Simon Peter (33 times) but mostly just Peter (139 times).
Peter was the first leader of Jesus’ followers, so calling him “rock” fits. Perhaps Jesus gave Peter a new name to preview his future as a leader.
Even more interesting is a play on words Jesus uses in Matthew 16:16-18. The implication isn’t apparent in most versions of the Bible, but the Amplified Bible captures it nicely (even throwing in some Greek to make sure we don’t miss it).
Peter (Petros, “a large piece of rock,” essentially a rock) gives a proclamation (Petra, “a huge rock like Gibraltar,” essentially the rock) saying Jesus is “the Christ, the Son of the living God.” Jesus affirms Peter’s words, declaring them to be the foundation on which he will build his church.
Peter is not the foundation, but his testimony is.