Sentence:  A ferlie (fairy) he spied with his ee, And there he saw a lady bright, Come riding down by the Elldon tree"— a lady who had "fifty siller bells and nine" on each lock of her horse's mane, and who <WSD>pillioned</WSD> Thomas the Rhymer, and took him, "red blude to the knee," thorugh the mirk, mirk night, to the place where "They saw neither sun nor moon." 
Meaning of the word marked with <WSD> tags:  To ride on a pillion.