Yes I know that this has come up on the forums before. I believe that the Beastmaster Ranger gets his animal companion at level 1. You may start throwing tomatoes if you wish. Even better, if there is an official ruling you could throw that would be even better.
My argument begins with the following:
1. Class / Archetype abilities that do not have a level listed are inherent to the class / archetype. Meaning that they are granted at level one (or the first level you take them at for multi-classing). Many examples exist in other classes, Wild Empathy is one from the base Ranger.
2. The text of the Beastmaster's Companion ability starts with this, "Animal Companion (Ex): A beast master forms a close bond with an animal companion. This ability functions like the druid animal companion ability except that the ranger's effective druid level is equal to his ranger level – 3."
3. The Druid Animal Companion text reads, "A druid may begin play with any of the animals listed in Animal Choices. This animal is a loyal companion that accompanies the druid on her adventures. Unlike normal animals of its kind, an animal companion's Hit Dice, abilities, skills, and feats advance as the druid advances in level. If a character receives an animal companion from more than one source, her effective druid levels stack for the purposes of determining the statistics and abilities of the companion."
4. I do not see anything that says the effective Druid Level limits the initial selection of an animal companion. Effective Druid Level does three things as far as I can tell. It stacks with other classes to determine total levels to give you animal companion. It gives you the number of levels you can divide between multiple companions. And it tells you when to level your animal companion to the next level / size.
5. My last argument (for now). A Beastmaster Ranger 1 / Druid 1 character would be a character level 2 with a Effective Druid Level of (-3) + 1 = (-2). Note that this would not be true of a normal Ranger 1 / Druid 1 character due to the fact that normal rangers don't get Hunter's Bond until 4th level. So until 4th level a Ranger has no Effective Druid Level, in other words value 0.... 0+1 = 1 for the Rng1 / Dru1, or 4+(-3)+1 = 2 for the Rng4 / Dru1.
Throw your rotten produce. :-)