The Beastmaster Prestige Class has the following class feature.
Animal Companion (Ex): A beastmaster gains the service of a loyal animal companion. See the druid class feature, pages 35-36 of the Player's Handbook. Treat the beastmaster as a druid whose level is equal to the beastmaster's class level + 3. A beastmaster can select one of the animals available to a 1st-level druid and then apply the modifications as appropriate for a 4th-level druid's animal companion, or she can select a typical version of one of the animals available to a 4th-level druid.
It states that you GAIN an animal Companion as if you are a 4rth lvl Druid. However, it also states that if you already have an animal companion, your levels stack to determine its power.
What does this mean for a 5th lvl Ranger who takes his first level Beastmaster? Does he gain an extra animal Companion as a 4th lvl Druid, and add his Beastmaster level to determine the power of both of those animal Companions?
Does he gain a new companion, but add the levels for only one of his pets?
Does he not gain a companion?
Is there another option I have overlooked?
Another Class Feature does state the following:
Extra Animal Companion (Ex): At 4th level, a beastmaster gains a second animal companion, chosen from the list of animal companions available to a 1st-level druid. Treat the beastmaster as a druid whose level is equal to the beastmaster's class level – 3 for the purpose of improving the animal companion's statistics (or of selecting an alternative companion at higher levels).
Thus implying that the Beastmaster still only has one animal Companion at his first level, independant of previous classes.
Best Answer
A beast master has one animal companion until 4th, then two animal companions until 7th, and then three animal companions until 10th. A 10th-level beastmaster has four animal companions.
This is not affected by how you enter the prestige class.
So, for your example of a 5th-level ranger (who has the animal companion of a 2nd-level druid), when he becomes a 1st-level beastmaster, he gains four effective levels in druid, so his (already-existing) animal companion becomes as strong as the animal companion of a 6th-level druid.
At the ranger/beastmaster’s 4th beastmaster level, this animal companion is now as strong as that of a 9th-level druid. He additionally gains a second animal companion, at his beastmaster level — 3 (read: 1st). In total he has one animal companion that is as strong as a 9th-level druid’s, and a second animal companion that is as strong as a 1st-level druid’s.
In the end, as a 5th-level ranger/10th-level beastmaster, he has four animal companions:
as strong as the animal companion of a 15th-level druid.
as strong as the animal companion of a 7th-level druid.
as strong as the animal companion of a 4th-level druid.
as strong as the animal companion of a 1st-level druid.
If the beastmaster were not a ranger, but rather a barbarian, he would be exactly the same except that his first animal companions would be as strong as the animal companion of a 13th-level druid, not 15th.
(As you might imagine, the actual usefulness of those low-level animal companions is seriously limited.)