To those of you wondering if the pupil being wider than taller lets you see more side to side than up or down – it doesn’t. Apertures don’t work that way. A horizontal pupil, a vertical pupil, and a round pupil give you the same range of vision, all other things being equal.
What it does affect is the region of the lens and cornea through which you see (a lot of people now claim that the differently-shaped pupils regulate which portion of the eye you let light in through, affecting the quality of the image as non-spherical and gradient index portions come into play) and the Modulation Transfer Function of the eye (the smaller a pupilo is in one dimension, the smaller the range of frequencires you can see, with the higher frequencies losing out. Contrarily, the larger it is, the more frequencies you can see. My argument for slit pupils on cats and snakes is that it lets them see extremely fine horizontal objects, while losing the resolution on vertical objects. So you can see, say, a mouse tail among grass stalks.)
WAG: based on the thread so far, it seems like your pupil slit runs perpendicular to the normal linear orientation of what you need to see. Cats need to focus on horizontal stuff, so their slit is vertical.
Mongooses eat snakes (and a lot of other stuff they can tike their time with). When a snake is horizontal it’s not much of a threat so you can take your time about getting at it. But when they fight back, the dangerous bits get vertical–you need horizontal slits.
Not really. From this wiki article “They typically avoid the cobra and have no particular affinity for consuming its meat”
This seems to be consistent with my observation, growing up in a small town in India.
Rudyard Kipling, you lied to me!!
From the same article:
[QUOTE=The Same Article]
Mongooses mostly feed on insects, crabs, earthworms, lizards, snakes, birds, and rodents. However, they also eat eggs and carrion.
[/QUOTE]
This seems to be consistent with my observation, surfing the internets up in a big town in Colorado far, far away from mongooses, cobras and India. 
But yeah, they eat snakes (and a lot of other things they can take their time with).
Kipling Vindicated (band name!)
Snakes does not mean cobras. Only the cobra gets vertical when striking - not other snakes contrary to your earlier posting - Inigo. There are more types of non-venomous snakes than cobras. I stand by my observation that the mongoose normally avoids cobras but if confronted it will fight back taking sometimes hours to kill the snake. And don’t get me started on Kipling.