Beorn

[The Might]

About the wizard part, he may not have really been aware about the connection between Radagast and Gandalf before really.

"I am Gandalf," said the wizard.
"Never heard of him," growled the man, "And what's this little fellow?" he said, stooping down to frown at the hobbit with his bushy eyebrows.
"That is Mr. Baggins, a hobbit of good family and unimpeachable reputation," said Gandalf. Bilbo bowed. He had no hat to take off, and was painfully conscious of his many missing buttons. "I am a wizard," continued Gandalf. "I have heard of you, if you have not heard of me; but perhaps you have heard of my good cousin Radagast who lives near the Southern borders of Mirkwood?"
"Yes; not a bad fellow as wizards go, I believe. I used to see him now and again," said Beorn. "Well, now I know who you are, or who you say you are. What do you want?"

So it appears that Beorn had no idea what wizards were or who Gandalf truly was.
Now, you may say that if had indeed learned so much from Radagast he would not call him only "not a bad fellow" and say they only met "now and again".
I admit that a bit more would be expected from Beorn when speaking of the one that passed this knowledge on to him, but still I see no theory being more likely than this one.