I think that if he was not afraid of Gandalf, he was close. Because Gandalf was now a White Wizard, that meant he was much more powerful than he had been before and that may have been niggling in the back of his mind.
I was thinking that maybe he rode the horse at the beginning just to see how much of a challenge the enemy presented to him. When he met Gandalf, maybe he decided that he needed a more formidable steed to face the White Wizard and switched to the Fell Beast.
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I like buying snacks from a vending machine because food is better when it falls. Sometimes at the grocery, I'll drop a candy bar so that it will achieve its maximum flavor potential.
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