Sforno on Exodus 4:3/ספורנו על שמות ד׳:ג׳

1וינס משה מפניו, for the staff had become a real snake and had assumed a threatening posture. This was totally different from the “snakes” produced by Pharaoh’s magicians that were unable to move. They only looked like the real thing, but were totally harmless. None of these sorcerers can produce something that is really alive, i.e. becomes part of nature. Our sages in Sanhedrin 67 commented on this phenomenon by saying: “not only can the sorcerers not produce small creatures, they cannot even produce big creatures such as a camel” This is why in the case of Moses, the Torah speaks of “the staff which had turned into a נחש, serpent,” whereas when the sorcerers’ work is described the Torah speaks of תנין. [at that point Aaron’s staff also turned into a תנין, not נחש, as G’d was not yet ready to demonstrate the superior magic Moses was capable of. Only when Aaron’s staff swallowed the “taninim” of the sorcerers was that point brought home to Pharaoh.]