Usually when I'm sketching live animals I like to start with a mechanical pencil and get a quick sketch for proportions and gesture. That makes it easy to erase if after just a few lines the animal has moved or gone off into a shelter. If I get a complete or near complete quick pencil sketch then I'll switch to pen or marker to refine. For the kangaroo, deer and giraffe I chose a burnt sienna waterproof marker because it made a statement about those animals and their coloring so right form the beginning the sketch was capturing something of the subject.
(It's also a good idea to wear a hat, sunscreen and a vest with lots of pockets to hold pens and what not.)
The rhino above was done with a fountain pen (Pilot Falcon Flex nib) and Noodlers Bulletproof black. That seemed like the right tool for all of his leathery wrinkled skin. The primates, who were constantly on the move were done with a Pilot Varsity fountain pen, which you can pick up in most stores for about $3, it has water soluble ink and can make beautiful tonal washes when you add a water brush.
The wolves moved around a lot and I was glad I had my stool because I had to spend a lot of time just watching them, and waiting for them to return to a similar position so I could continue the sketch.
The red wolf was my last sketch of the day and his head gave me fits. I finally had to look at a photo on the internet and use gouache and markers to get it right.