Interestingly, Monotype created cheap substitutes for not only Helvetica, but all the other proprietary fonts Adobe has included with PostScript. These were created at the request of Microsoft for inclusion with its PostScript clone, TrueImage, and also included with Windows and Microsoft Office.
Of these other substitutes, Book Antiqua looks nearly identical to Palatino. The designer of Palatino, Herman Zapf, has been known to do off-name versions of his own typefaces for other foundries (notably Bitstream), but in this case he had nothing to do with it, other than being copied. [Interestingly, Microsoft now includes a very good—and fully licensed—version of Palatino with Windows. —MS]
On the other hand, they created substitute fonts for two of his other typefaces, Corsiva for Zapf Chancery and Monotype Sorts for Zapf Dingbats.
As for the others, they are similar to Arial in that they are loosely based on typefaces owned by Monotype reworked to fit the proportions and weight of a specific non-Monotype font.
Century Gothic is Monotype’s Twentieth Century redrawn to match the weight and proportions of ITC Avant Garde Gothic. (Twentieth Century is Monotype’s version of Futura.)
Bookman Oldstyle is the original Bookman (late 19th century, ATF) redrawn to match the weight and proportions of ITC Bookman, including its cursive italic. The original Bookman had a slanted roman for italic.
Century Schoolbook is simply the earlier design upon which New Century Schoolbook is based, which both Monotype and Linotype licensed from American Type Founders. The two are virtually indistinguishable except for the extra weights offered in the Linotype version.
Times New Roman was developed originally by Monotype in the 1930s. Linotype’s Times is actually licensed from Monotype. In this case, the Monotype version is obviously more authentic, though the differences are extremely subtle.