There is generally a lot of confusion when it comes to ticks and which ones are dangerous, and when or if you need to rush your pet to the vet. Here are two examples in the photos - The light grey/green one is a Paralysis Tick and your pet should be taken to the vet immediately. The dark coloured one is a Bush Tick, which is generally harmless to your pets.
You can see the difference in the placement of the legs in these two examples :
The Paralysis Tick has it's legs very close to it's head at the front in a 'V' shape.