Shop around locally (Craigslist) or online (Amazon, eBay, here on POTN in the Classified area) for an EF 28-135 IS.
There are tons of them around that were sold in kit with 40D and 50D, just gathering dust until their owners sell them off.
It's a great multi-purpose lens that can be bought lightly used for a big discount off the usual price. Sells for $450 new, plus $30 for the hood. I bought someone's kit lens, practically unused, with hood and a $60 B+W filter for all of $250, including shipping. Found it on Amazon, if I recall. I got a great deal, it usually sells for closer to $300 in this condition. But even that is a great deal for a lens that can go head to head with 24-70L and 24-015L for image quality.
It's a decent, usable range of focal lengths, including all the "traditional portrait" lengths, though not wide. EF so it will fit older DSLRs and even full frame or film. USM so it's got fast, quiet, accurate focus. Decent mid-grade build quality ("USM" gold stripe... not L red stripe, but not entry-level plastic toy-like either). Good close focus. And IS to help keep your shots steadier at a jog or trot.
The downside? It's not sealed against dust as well as L-series (which is one reason I usually use a 24-70L instead). And it's an f3.5 to f5.6 lens, so there's limited opportunity to blur down backgrounds or shoot in really low light (f2.8 is another reason I usually use the 24-70L and keep the 28-135 as a backup/loaner/walkaround lens). And it's not wide at all. On an APS-C size crop sensor it will behave as a standard to moderate telephoto lens.
Want wide too? Sometime pick up a used Tokina 12-24/4 first version for about $400 and you'll have a very complete, decent quality kit for minimal outlay of money.
I've taken (and sold) thousands of images of horses and their riders with 28-135.