Honestly, if your goal is to be shooting equestrian events the 70-200mm F2.8 is a no brainer. It is a FANTASTIC sports lens that also happens to be an amazing portrait lens.
One other option you might want to consider though is Sigma's 120-300 F2.8, it also makes a good portrait lens and has even more reach for sporting events. They just announced a new one so the last generation may have some nice used prices when it comes out. ([link])
Granted both those lenses are very big, however, but i'd say if you are looking for a smaller option for shooting pets your 50mm F1.8 should more than suffice.
With a crop body I'd go with the canon 17-55 F2.8 is. I love the lens, it's optically as good as an 'L' lens, and has roughly the same focal length as the 24-70 does on a full frame body (entirely the point ) with the added bonus of IS!
The 70-200 F2.8 IS is an awesome lens, and is a firm favourite of mine, but on a crop body, it might just be a bit long for the closer in stuff! But that said, I wouldn't want to put you off it as it really is that good, and the extra stop is well worth it! I'd try and hire one out to see what you think of it! The only down side is it's a weighty beast! I know when I've had it swinging off my shoulder for 12 hours!!!
I hadn't considered the 17-55 2.8. Just had a quick read-up and it sounds fantastic, and the price is brilliant! The only thing I am worried about is that at some stage in the next few years I will no doubt be moving up to a full frame and then would need to purchase the 24-70 for that camera. That being said, it seems to be a great alternative to the 24-70 for the short-term, so I will certainly look into more. Thanks for bringing that to my attention.
Yes, I got to play with a 70-200 2.8 at a show couple of weeks ago and it was incredible! I did find it more difficult to work with in tighter situations, but if I also had a 24-70 (or 17-55) then it wouldn't be a problem. The weight is what concerns me - with pet photography you need to be able to move around a lot and often use a camera with one hand while getting the dogs' attention with a treat or toy in the other - after using it for a few hours at a show the other day I don't know how well I would cope at a shoot with such a heavy lens. It is such a high quality lens however (the images it is capable of producing is simply mind-blowing), and the weight vs extra stop is a difficult choice to make!
Many people on forums will tell you to get equipment with upgrade in mind... I know when I was in that situation, I really didn't know when that time would be, so my ultimate advice is buy what you need now, not what you need in the future... If you look after your kit, it will always have a good second hand value!
As for gaining attention, I have a head band with glittery balls on springs, you only have to wiggle your head to get them moving, and certainly grabs the attention of children and babies, to I imagine it likely would with dogs to! Both hands free!
The head band sounds great - I'll have to give that a go! Usually I've just been using treats and/or squeaky toys, but having my hands free to use the camera would be really useful. Thanks for the tip.