Oxford Dictionary of Celtic Mythology:
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[Scottish Gaelic, bear, serpent, wild beast; dragon (?)]

A word in early Scottish Gaelic narratives for an undetermined savage creature. It may be a translation of the Norse word for ‘bear’, but it may also mean ‘thunderbolt’. The beithir may have a long tail, but it never appears to be the fiery winged dragon of Germanic tradition. In more recent oral tradition the beithir is a class of fuath who haunts caves and corries (narrow circular valleys with high walls). It may also imply ‘lightning’ or ‘serpent’. A mountain south of the entrance to Glencoe is named Ben Vair or Ben Vehir, thought locally to commemorate a beithir who took shelter in Corrie Lia.
So this is the mysterious Monster I have chosen. I think I can wrap my mind around it!! Cue the maniacal laugh