Sforno on Exodus 7:12/ספורנו על שמות ז׳:י״ב

1ויהיו לתנינים, they looked and felt like these kinds of monsters. They were not able to move like these monsters move, or even not at all.

2ויבלע מטה אהרן את מטותם, this would demonstrate that only the G’d in heaven can grant life to inert phenomena, something Pharaoh’s sorcerers were unable to do.