Bizarre request - is there a rule for this?

My player is a Druid. He asked if he could turn his camel into a giant ape and train it to attack. Can a druid alter the shape of an animal anyway? FYI, my answer to this is a 90% NO, you cannot. It's the other 10% of me that would like to see what happens. Camels are already large. How could he do it? And, if you feel like it, why should I ban it or allow it?

A Druid cannot do this simply by virtue of being a Druid.  Just because they can transform into an animal does not mean they can transform other animals.  

However, there is a way for them to do this - the Polymorph (and at much higher levels, True Polymorph) spell accomplishes this goal, with defined rules on concentration, duration, etc.  That is already a spell on the Druid spell list, so you should just tell him that is the mechanism by which he can accomplish this.  Then you just play the spell exactly rules as written.

Well, the Druid class, as written, doesn't have any such abilities. There are spells like Polymorph that can change something's form temporarily, though technically that spell cannot change a Camel into a Giant Ape due to the CR limitation. 

But basically no one follows the D&D rules exactly in all situations; if you feel like it'd be fun for this to work, then go for it.

Some things you should consider are:

• As written, a Giant Ape is a much more powerful creature than a Camel. Depending on the level of the player characters, having one around could drastically unbalance combat encounters, so if you allow this, you might not want to use the official Giant Ape stat block.
• Does the player want this to be a permanent change? Or something more like Wild Shape where it lasts for a certain amount of time and can be done a certain number of times per day?
• Is this going to be limited to this one creature, or is the player asking to have a broad ability to transform animals into other animals all the time?

Nope, druids can't do that without higher level spells.

However, if this is something the character is invested in, you could give them a wand of polymorph and let them go wild with it.