are purchasable animals OP?

I have recently discovered that you can buy a large variety of rather strong creatures for very cheap amounts. A character that is moderately specialized in handle animal can easily get a +15 by level 3. And for the price of a +1 weapon, they can instead get a Mastodon. This monster of a creature has 14 HD and can reasonably be combat trained by the character. This monster has a +20 to hit, and does a LOT more damage than anyone at that level should be able to pull off. This problem could be made even worse if the character making the purchase has access to a spell like animate dead. These are much stronger than creatures at the level it would be castable, and make for cheap strong minions. (ignoring story consequences.)

Is there something I am missing about these animals and why they are priced so low in comparison to other equipment and gear that is much less powerful?