In the first book (Dune), the only description we have is of two of their agents, who're described as "men" with no indication of non-humanoid features:
He and his companion pushed through to the barrier lances, which were raised at a nod from Paul. The two men stepped out and the taller leveled an arm at Paul, said: “You may very well be under embargo for your—”
“If I hear any more nonsense from either of you,” Paul said, “I’ll give the order that’ll destroy all spice production on Arrakis … forever.”
“Are you mad?” the tall Guildsman demanded. He fell back half a step.
“You grant that I have the power to do this thing, then?” Paul asked.
The Guildsman seemed to stare into space for a moment, then: “Yes, you could do it, but you must not.”
“Ah-h-h,” Paul said and nodded to himself. “Guild navigators, both of you, eh?”
It's not until the second book (Dune Messiah) that finally we meet a true Steersman, one of those capable of piloting a heighliner;
Scytale shifted his gaze from Irulan to the tank, inviting the
Princess to share his viewpoint. She would, Scytale knew, see Edric as
a repellent figure: the bold stare, those monstrous feet and hands
moving softly in the gas, the smoky swirling of orange eddies around
him. She would wonder about his sex habits, thinking how odd it would
be to mate with such a one. Even the field-force generator which
recreated for Edric the weightlessness of space would set him apart
from her now.
“You know what is said of Alia?” Scytale asked, probing.
“What do you mean?” Again, the fish-man was agitated.