What was the most terrifying animal Lewis and Clark encountered?

Answer 1

According to Lewis, the most terrifying animal was the grizzly bear:

"...these bears being so hard to die rather intimidates us all; I must confess that I do not like the gentlemen [the bears] and had rather fight two Indians than one bear."

On May 14th,1805 some of the men had another encounter with a bear:

"...one of the party wounded a brown bear very badly, but being alone did not think proper to pursue him. In the evening the men in two of the rear canoes discovered a large brown bear lying in the open grounds about 300 paces from the river, and six of them went out to attack him, all good hunters; they took the advantage of a small eminence which concealed them and got within 40 paces of him unperceived, two of them reserved their fires as had been previously conscerted, the four others fired nearly at the same time and put each his bullet through him, two of the balls passed through the bulk of both lobes of his lungs, in an instant this monster ran at them with open mouth, the two who had reserved their fires discharged their pieces at him as he came towards them, both of them struck him, one only slightly and the other fortunately broke his shoulder, this however only retarded his motion for a moment only, the men unable to reload their guns took to flight, the bear pursued and had very nearly overtaken them before they reached the river; two of the party betook themselves to a canoe and the others separated an concealed themselves among the willows, reloaded their pieces, each discharged his piece at him as they had an opportunity they struck him several times again but the guns served only to direct the bear to them, in this manner he pursued two of them seperately so close that they were obliged to throw aside their guns and pouches and throw themselves into the river altho' the bank was nearly twenty feet perpendicular; so enraged was this anamal that he plunged into the river only a few feet behind the second man he had compelled take refuge in the water, when one of those who still remained on shore shot him through the head and finally killed him."