Yes, ovis and canis are i-stems. My interpretation of this poster's problems was that he did not understand 3rd declension generally speaking (he was confused about the nom. and gen. being the same). There is no real way to "explain" i-stem without going into mind-numbing rules, one of them being that nouns that end in "is" and have the same number of vowels in the nom. and gen. are i-stem. If the teacher wants him to do that, we can roll, but I would guess he does not, since Lingua Latina eschews niceties such as this. (I am assuming he is part of the Lingua Latin New York group).
But I shall ask, Luke...how much detail are you looking for? There is a type of 3rd declension called i-stem that change the endings slightly. The big thing is that gen. pl changes to -ium instead of -um. Acc pl can sometimes be -is but not necessarily and neut can add an i before the a in 3rd declension i-stem. But these changes are slight, and unless you are composing, not that big a deal. I have had students for three years whom I never mentioned I-stems to, and they were never confused when they came across one in a reading.
Oh yeah, neut i-stems have -i in the ablative.
When a true genius appears, you can know him by this sign: that all the dunces are in a confederacy against him. ~Swift