Types of familiars

If I have a Warlock with Pact of the chain, I can choose one of the normal forms, but what is that?  Is it anything with a CR 0 or less?  

Can I choose a pack animal or a Mastiff?  If so can I use my familiar to carry my gear like a pack animal?

Can it be a blink dog or even a rust monster or is that a special form?

 The animal form you choose when you cast Find Familiar  are Bat, Cat, Frog, Hawk, Lizard, Octopus, Owl, Rat, Raven, Spider, Weasel, or another Beast that has a Challenge Rating of 0.

Mastiff No because it's not a Beast with CR 0.

Blink Dog No because it's not a Beast but a Fey.

Rust Monster No No because it's not a Beast but a Monstrosity.

So it appears that there is a goat or sheep or a skeleton that can probably be used as a pack animal.

Is there any issue with using them like a pack animal assuming the aren't scouting or something else?   As for combat capabilities, the need for them to take my characters attack action has left me thinking it is not worth using them for direct combat.

There are also CR 0 beasts that are special case creatures and may technically fit the CR 0 Beast restriction but aren't intended to be allowed, such as a Giant Fly (a transformed magic item) or Haungharassk and Onyx (unique adventure NPCs).

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Also, note that at level 5 you can take the Eldritch Invocation "Investment of the Chain Master", which lets you use your bonus action to have your familiar attack.

Agreed, assuming you are not doing the invocation of two minds which needs a BA to continue.  

Quote from SmiteMakesRight_3_5 >>

There are also CR 0 beasts that are special case creatures and may technically fit the CR 0 Beast restriction but aren't intended to be allowed, such as a Giant Fly (a transformed magic item) [...]

 I agree with you, but... Find Familiar: Giant Fly :)