Looks like Laggan was known as Logane/Logyn early on.

Then it became Logachnacheny later Laggan-Choinnich (logach-na-cheny is the parish name of the old church = hollow of kenneth).
It is mentioned in 1239 as Logynkenny (R.M.), and Logykenny shortly before, as Logachnacheny and Logykeny in 1380, Logankenny in 1381 (all from R.M.), and Lagane in 1603 (H.R.) The Gaelic word " lagan" is the diminutive of " lag," a hollow.

The earlier name might have sounded like Logane/Logyn or just Logy as well, in the pictish or brythonic, and then have been gaelicised to a gaelic word sounding similar, as was the case at Loch Lochy nearby. That was the usual practice. And so something sounding like logy became became Logaidh (hollow, tap, forelock) first, possibly, then Laggan anglicised. That or the original name was wiped out, but the practice was to pick a similar name.

Name itself no clue then. Loch Oich etymology worked out better than this for finding its monster. Sometims it works. Loch of the Lords is a tempting brythonic etymology in light of the king burials, and a few other things which are all circumstantial evidence.

Getting back to a possible monster for Laggan Loch. If I had to guess, from other names in the area and stuff like that, I'd say if it had a serpent originally it was the pictish white serpent like the one at Tummel.