The pen is a Wirt slip-cap dropper filler in black hard rubber with a gold filled "filigree" overlay. It's the sort of pen that would normally be worth several hundred, but this one carries an auction estimate of $15,000 to $25,000 thanks to engraved inscriptions that identify it as a gift from Theodore Roosevelt to Pat Garrett, including the date of December 20, 1901, when Roosevelt nominated Garrett as customs collector of El Paso. According to the catalog entry, this pen previously appeared at auction in 1996 and was shown in a book on the Old West first published in 1992.
The problem is that this pen cannot date to 1901. The nib and feed are of a form that Wirt did not adopt until a couple of years later (US patent 724984, applied for and issued in 1903), though the exact form of the feed appears to be later yet (see US patent 978420, applied for in 1908 and issued in 1910). And even if the nib and feed are not original to the pen, the trefoil pattern filigree of the overlay is a design of distinctive form that was only introduced in the 'teens -- several years after Garrett's death in 1908. What Wirt was actually making in 1901 is nicely illustrated by the 1901 and c. 1903 catalogs in the PCA Reference Library (and the latter catalog may well be later than the listed date estimate). Unfortunately we don't currently have comprehensive documentation of Wirt's offerings over the following decade or two, but since Wirt like virtually all pen companies subcontracted overlay manufacture, designs can easily and precisely be traced across the pen industry at large.
The interesting thing is that the engravings don't appear to be new, though they are suspiciously crude for what purports to be a presidential gift. This isn't the first pen that's turned up with an old inscription that wasn't as old as it purported to be -- I'm thinking of a Waterman silver overlay safety pen that recently turned up at auction with a Stanford family inscription -- so this sort of "enhancement" seems to have been going on for some time. The puzzling thing is that the engravings wouldn't always have obviously enhanced the pen's market value. That wouldn't have been the case with this pen, of course, but it's hard to imagine the Stanford inscription attracting a collector's attention at the time it was done. One can speculate that in at least some cases a pen might have been engraved well after the fact to reflect family lore about its history. Yet as experienced antiques dealers and experts well know, such lore can be unreliable. Even where the core story may be true, over the years it all too often may end up attached to a different item. In the case of this Wirt, however, we don't even seem to have any provenance tying it back to Garrett's family or friends. It's yet more speculation, but the Waterman box in which the auction pen is being offered could actually be for a pen purchased in 1901.