We've seen various different animals in the different parts of Costa Rica we've been to. In basically all the towns we've seen a ton of dogs, and a few cats too. The dogs we've seen generally just wander the street, but a lot of them aren't strays. Many of them have owners, and sometimes collars, but are left free to wander for the most part. We were warned at one hotel that most of the dogs wandering the street have fleas and to be careful about petting them, but some of them are so cute when they come looking for attention.
In the mountainous areas of Costa Rica we've seen several different types of cows and there seem to be a lot of cow herds in the hilly regions near La Fortuna, Volcan Arenal, Monteverde, etc. On the road to Monteverde we even saw some cows being used to pull a small cart. Horses are also very popular, especially in small towns and areas where the roads aren't in great condition. We've seen people in various areas riding horses along the roads or highways, and horses tied up at small shops or restaurants. On the road to Monteverde, which was pretty bumpy and definitely would not be great in any type of sedan, we saw several people riding horses, which we figured would probably be easier than trying to take a car.
By the beach we mostly saw tons and tons of crabs. The beaches were covered in crabs that dug holes in the sand, and even on the roads towards town and our hotel we saw several fairly large crabs that had dug holes along the side of the road or in planters. We even got three or four crabs that snuck into our hotel room in Coco, and we took them back outside. At Playa Hermosa we found a river that ran into the ocean and the treed area around the river was swarming with hermit crabs. You really had to watch where you were walking to make sure you weren't stepping on any in that area.
Around Coco and on the road to Monteverde we also saw a few iguanas. The iguanas we've seen are all gray-ish brown and generally fairly small, at least for iguanas. They blend in really well with the landscape and are really fast when they want to be - too fast for me to get any pictures of them so far. The iguanas here are pretty different from the ones I saw in Mexico; the iguanas in Mexico were generally quite large (usually bigger than my cat Felix for anyone who's seen him) and they tend to just lay in the sun, even if you walk up to take a picture of them.