I have never been to Costa Rica, so today I am sharing what I'm teaching myself, and you're welcome to correct me if you know more.
But first, the Chestnut-backed Antbird:
photo credit to: Jorge Chincilla
Puffy little thing. Common in the understory of wet forests. Like most antbirds, it doesn't actually survive solely by preying on army ant swarms, it just forages on them opportunistically.
This is a male, probably. Like I said, I'm not completely familiar with the species, but the females are said to have brown coloration on their breast and belly, and males have black.
Other things I should look out for when I'm down there are venomous snakes (like bushmasters) and jaguars (I really want to see one, but if I couldn't live to talk about it, it wouldn't be worth seeing), all manner of really awesome bugs (check out the list here), and bird heaven. I will be in tropical wet forest.
From what I've seen of species maps, Costa Rica is apparently home to two species of bushmaster, the Black-headed bushmaster and the Central American bushmaster.
Black-headed bushmaster (Lachesis melanocephala)
Central American bushmaster (Lachesis stenophrys)
Not a super great shot of the body, but the difference in head coloration is mainly what you need to see. Genus Lachesis consist of three species of venomous pit vipers (not shown here is the South American bushmaster). Bushmasters are the longest venomous snakes in the western hemisphere, and longest viper. The venom can be fatal, but usually only causes sever pain, vomiting, nausea and the chills. Thankfully the snake is most efficient as a nocturnal predator and it's contact with humans is limited because of this. With regards to the myth that they hunt in pairs and are attracted to fire, I've found millions of statements denying both.
Another snake to fear would be the Fer-de-lance, or terciopelo (Bothrops asper). Another viper. You can research that one on your own.
As far as snakes in Costa Rica go, unless you know your stuff, leave them alone. This is directed at myself. Though hopefully I will meet an expert down there and benefit from their knowledge and wisdom.
As far as bugs go, I really want to see a Hercules Beetle (Spanish name: Cornizuelo, Dynastes hercules), and here is why:
That thing is awesome. I want to hold it right by my face and take a very cliche photograph with it.
As far as mammals go, I haven't got anything specific in mind, but I will be on the lookout for pretty much everything. I'm hoping to procure a couple field guides. Not sure which are the greatest yet, but don't worry, I will do my research.
Also, on a related note, I'd like to make sure everybody knows how to differentiate between a venomous snake and non venomous snake. The guideline I'm including is a rough guide, there are exceptions. The part to pay most attention to is the head shape, because that's usually what you can see from a ways away.
Also, it says U.S. snakes, but it's pretty universal, and this chart also doesn't include sea snakes, simply because the one with sea snakes wasn't as well drawn.
Use wisely.