Chambers gives the past tense as 'slayed'.
It also listed several possible derivations from Old English, Old Norse, Gothic and German so it is probably not surprising that it looks odd.
"And hast thou slain the Jabberwock?" 'Lewis Carroll' evidently thought, ufishly or otherwise, that 'slain' was correct. Chambers? Hah! 'He slayed', yes.'He slew', yes. 'He slain' obviously no. 'He has slain', yes. 'He has slayed' , possibly, but not as common as 'slain'
Seattle Slew was a winner of the Triple Crown. His name was meant to be Seattle Slough but was changed to Slew so it was easier to say [according to Wiki] Two towns in one name would be reason enough, but apparently 'slough' is a water channel local to the area where he was.
A mriner would say a mast 'slews', when it twists around in its step, its holder at the base, but we say boats and planes 'yaw' when they swing to one side, turning about their vertical axis, thetaliesin
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