I have consistently noticed that sabre cats and foxes in Skyrim do not respawn in a given location when killed. I looked in the CK and discovered that all varieties of sabre cats and both varieties of foxes do not have the respawn box checked (see below). I also noticed the same on wisp mothers, which I also have observed do not respawn as they should. Other creatures such as bears, wolves, deer, etc. seem to respawn fine in the game (from my testing) and they have the respawn box checked in the CK.
Skyrim.esm
EncSabreCat > Respawn flag is not checked -- (corrected in USKP 2.0.0)
EncSabreCatSnow > Respawn flag is not checked -- (corrected in USKP 2.0.0)
EncFox > Respawn flag is not checked -- (corrected in USKP 2.0.0)
EncFoxArctic > Respawn flag is not checked -- (corrected in USKP 2.0.0)
WispRace > EncWispMother > Respawn flag is not checked -- (apparently intentional)
Dawnguard.esm -- Under Actors:
DLC1EncSabreCatVale > Respawn flag is not checked -- (corrected in UDGP 2.0.0)
DLC1DarkFallEncSabreCat > Respawn flag is not checked -- (corrected in UDGP 2.0.0)
Dragonborn.esm -- Under Actors:
DLC2EncBoar > Respawn flag is not checked -- (corrected in UDBP 2.0.2)