mhmm, it does seem to be only the plants. i usually see unicorns, satyr, and pegasoi wandering around in certain embarks. although, the plant thing still irks me, i mean, an attempt to get sun berries and sliver barbs (and whip vines) in one embark is why i even started my triple biome setups i always do now
as for the plural of pegasus, yeah, if we're going english it's most likely pegasi, but it was originally a greek word, and i think their pluraling ended in 'oi' for it. but that's just something i remember reading, prob'ly in these forums, even. or we could just join them to the whole 'octopi/octopuses debate that is still going on in the wider world.
also, with the adding [PET] to unicorns, make sure they don't have the [PET_EXOTIC] tag already in somewhere. i'm sure things wouldn't be fun if it had both tags active. again, i've removed all exotics and done so many other minor modifications to most of my animals that i really don't remember exactly what actually starts with what tags... (and, note...if it's at least semi-sentient, if you give it the pet tag, you also want to add noeat and nodrink, if you're planning on pasturing it...i always forget that with each pet-race until a gremlin or satyr or animalman dies of thirst)