Most fountain pens can travel in a jacket pocket, but some of us like something so portable that it can fit into the smaller pocket of a pair of trousers. Here's a handy guide to getting started if you set out on that path.
Firstly, recognise that the pen is going to be short - necessarily so, as it has to fit in a cramped space and not snap when you sit down. That means filling options are likely to be limited to short 'international' cartridges or very low-capacity converters. It also means that the pen will too short to use without 'posting' the cap onto to the back end. If you don't like either of those restrictions, read no further - a pocket pen probably isn't the EDC for you.
If shortness of stature is no barrier, there is actually rather a lot of variety out there. Should you wish to try out the concept of a pocket pen without investing heavily, the simplest approach is probably just to go straight to the affordable, plastic Kaweco Sport with a reliable steel nib. They work well, they're pretty robust and despite the reasonable price tag they have quite decent endurance - and an army of committed collectors.
Beyond that ubiquitous starting point the options for upgrading largely come down to materials and nib sizes. The most widespread tougher materials are metals; aluminium if you want it light, brass or steel if you want it heavy. Kaweco make versions of the Sport in all these metals, as it happens, so if you like that shape you're well-provided for. There are also some fairly solid 'tributes', like Ensso's amusingly-named XS, which is of course tiny. Generally, a pen milled from a bar of tough metal is going to last for decades, so it really is a matter of personal taste. Some find that brass leaves a smell they dislike on their hands, but interestingly by no means all do.
If you have the budget for a metal Sport you can also start to consider different nib sizes. The Sport, like it's even tinier cousin the Lilliput, employs a short, narrow #5 nib, Bock's O60. In the Lilliput that's really the only thing there's space for, but if you prefer a bigger nib with a bit more natural bounce then a #6 has a lot going for it. Funnily enough Kaweco have endeavoured to cover this base too, with the shrinkable Supra (in brass or steel), but here they do now have some stiff competition from Schon DSGN, whose formsfollows-function Pocket Six offers no unnecessary quirks but still looks terrific. Another (British, this time) pen maker is currently experimenting with a pocket pen design using a nib size in between these, the Bock 076, and more on that to follow as it develops. But for now, the best advice is probably to start with affordable plastic and a sensible steel nib, and work up from there. Dangerously, most of the metal options have screw-in nib units, so it's quite feasible to go a bit crazy and move up to gold, too. That's possibly a silly thing to carry around in your beaten-up jeans - but by golly it works!