I have a ranger, and I am standing next to a newly discovered pet, but it doesn't give me the option to "charm". Is there something special I have to do to get this option to pop up? or is this possibly just a bug?
Are you sure the pet in question is charmable?
You need to find a "Juvenile" version of the animal - not just any random mob. "Hatchling" or "Young" does not count; it must be a juvenile!
Be careful, because you might also be trying to tame another player's pet - if a player has not named their pet, it will remain "Juvenile X", and may give no other indicator that it belongs to another player.
Finally, you may have already tamed one of those animals. Rangers are limited to 1 of each unique pet - you can't tame multiple pink moas, no matter how cute you might find them.