Shell Shield

On autocast, your turtle will use this ability at <50% health.

Problem is that you may find your turtle may not be able to hold aggro on a mob with this ability active while you fire away at it.

Turtle as main tank *can* be done. It's unconventional and requires a bit of "out of the box" thinking but it can be quite effective. To date with my BM hunter I've had my turtle main tank Zul'Farrak and Sunken Temple in a party consisting of a priest, enhancement shaman, mage, and me. Only on one or two occasions did the turtle lose aggro; mainly due to the mage firing a moment too soon before the turtle had a chance to establish aggro.

Shell Shield is the key to this in my opinion. I currently respec my turtle depending on the instance to be run. Bite/Growl/Shell Shield, Max the health, max a particular resistance (depending on the dungeon), then max armor as much a possible (given talent points). Bestial Wrath and Intimidation do the rest in terms of aggro generation.

It's not without some work, but if you can manage it you can have a good Tank and a top notch DPSer filling only one player slot this way. It annoys me constantly when people trash turtles when I know I've been able to so great things with mine.