I ran into this idea while making a high level Druid NPC.
1. Be a Druid out to 20th level, or at least a class that continues to give you levels for the purpose of your Animal Companion.
2. Keep a first level animal companion until you reach level 20. It will get +12 HD, +12NA, +6 STR/DEX, Evasion, Multiattack, Improved Ev.
3. Cast a Maximized Awaken on the animal companion. This gives it a further +2HD, as well as +3 CHA and 18 INT. The animal becomes sentient, can no longer be your animal companion, and is friendly towards you.
4. Get a new companion from the level 1 list. It will get +12 HD, +12NA, +6 STR/DEX, Evasion, Multiattack, Improved Evasion, etc.
5. Repeat steps 3 and 4 as much as you'd like.
End result: A lot of animals which is friendly towards you, has 14 more HD, 12 more NA and 6 more STR and DEX than normal (not counting the stat ups and feats from all the extra HD!), and is a lot smarter than the average PC/NPC. Whether or not your DM would let the animal keep the stat increases and/or feats from being an animal companion is iffy, but I'd let it through. Basically, you can make yourself an army of very large animals that are willing to help you out. Thoughts?