In general, the law is not based on the species, it is based on the use of the animal. This gives rise to the difference between food rabbits and pet rabbits, and so on. The definition or applicable offenses are spelled out in each state's criminal code. In Washington, Chapter 9.08 RCW covers "Crimes relating to animals", and 16.52 RCW covers "prevention of cruelty to animals". 9.08.065 defines a "pet animal" as
a tamed or domesticated animal legally retained by a person and kept
as a companion. "Pet animal" does not include livestock raised for
and then makes it a crime to steal a pet animal (this is in addition to regular laws against theft that applies to any property). So if you steal someone's pet goat, that's two or more crimes, but if you steal a meat goat, that's one less crime.
The main anti-cruelty laws are in 16.52. Distinctions may be made between between domestic animals and generic animals, or between livestock and others, or food animals and others, so it just depends on the action being forbidden. RCW 16.52.205 says you commit first degree animal cruelty if you
intentionally (a) inflicts substantial pain on, (b) causes physical
injury to, or (c) kills an animal by a means causing undue suffering
or while manifesting an extreme indifference to life, or forces a
minor to inflict unnecessary pain, injury, or death on an animal.
There is no other provision allowing you to torture any animal, not even a backyard rat. It is generally illegal to poison animals, but RCW 16.52.190 allows euthanizing by poison, or pest-eradication by poison (insects are animals too). Thus it is legal to poison a pest rat but illegal to poison a pet rat. There is also a general exception, in 16.52.180, that
No part of this chapter shall be deemed to interfere with any of the
laws of this state known as the "game laws," nor be deemed to
interfere with the right to destroy any venomous reptile or any known
as dangerous to life, limb or property, or to interfere with the right
to kill animals to be used for food or with any properly conducted
scientific experiments or investigations, which experiments or
investigations shall be performed only under the authority of the
faculty of some regularly incorporated college or university of the
state of Washington or a research facility registered with the United
States department of agriculture and regulated by 7 U.S.C. Sec. 2131
This allows you to kill chickens for food, and might be construed as allowing you to feed rats to your snake (the law does not say "used as food for humans"). The definition of 1st degree cruelty also has an exception that "Nothing in this section may be considered to prohibit accepted animal husbandry practices" (however, keeping an animal as a pet or educational object does not constitute "animal husbandry" in the ordinary meaning of words). There is no clear statutory division in Washington between feeding rats to reptiles, and feeding dogs to reptiles, and if you were to feed kittens to your monitor lizard, you might well get arrested.
Idaho animal cruelty law also forbids cruelty to any animal:
Every person who is cruel to any animal, or who causes or procures any
animal to be cruelly treated, or who, having the charge or custody of
any animal either as owner or otherwise, subjects any animal to
cruelty shall, upon conviction, be punished in accordance with section
25-3520A, Idaho Code.
and cruel(ty) is
(a) The intentional and malicious infliction of pain, physical
suffering, injury or death upon an animal; (b) To maliciously kill,
maim, wound, torment, deprive of necessary sustenance, drink or
shelter, cruelly beat, mutilate or cruelly kill an animal; (c) To
subject an animal to needless suffering or inflict unnecessary
cruelty; (d) To knowingly abandon an animal; (e) To negligently
confine an animal in unsanitary conditions or to negligently house an
animal in inadequate facilities; to negligently fail to provide
sustenance, water or shelter to an animal.
Again, feeding a rat to a snake is not intrinsically malicious, nor is feeding a puppy to a turtle intrinsically malicious. A distinction can be made under the related law on torturing a companion animal (an extension of the original anti-cruelty law):
A person is guilty of the offense of torturing a companion animal if
he tortures a companion animal as defined in this chapter.
where "companion animal" is defined as
those animals solely kept as pets and not used as production animals,
as defined in this section, including, but not limited to, domestic
dogs, domestic cats, rabbits, companion birds, and other animals.
This gives a basis for distinguishing feeder rats from feeder puppies.
Torture is then defined as
the intentional, knowing and willful infliction of unjustifiable and
extreme or prolonged pain, mutilation or maiming done for the purpose
of causing suffering. "Torture" shall not mean or include acts of
omission or of neglect nor acts committed unintentionally or by
accident. "Torture" also shall not mean or include normal or legal
practices as provided in section 25-3514, Idaho Code.
And thus it is not clear that feeding an animal to a turtle counts, since the purpose is to feed the turtle, not to cause suffering. The exceptions spelled out in 25-3514 might be applicable, but there are not clearly applicable. One exception is "The humane slaughter of any animal normally and commonly raised as food, for production of fiber or equines" – perhaps using an animal as feed for another animal can be "humane slaughter", perhaps rats are "normally and commonly" raised as food, unlike dogs.
It is not currently against federal law to slaughter cats and dogs for meat, but there is a bill in Congress which would make it so. The bill has 245 sponsors in the House, so it is likely to pass.