Sub-Roman Britain (Cavalry etc)

[Agraes]

I missed the connection between the Picts and Britons, other than occupying the same island. I was under the impression that the Picts were a distinctive people (as much as any tribal group could be different from its neighbors) distinguished from both the Briton tribes to their south, who became Romanized under the Empire, and the Dalriada Scots, who began to invade/settle/whatever to their west about AD 600. Is that not true?

The interactions between those different groups is not that simple. The tribes of the Novantae, Selgovae, Damnonii and Votadini were not officially weathin the empire, except when the Antonine wall was occupated for a few decades. It seems that they were allied to Rome at some point as some of their leaders have latin names in the geneaologies (Tacitus, Paternus Pesrut, Aeternus, etc.). In the Early Medieval period, they are considered as Britons, not Picts. However as much they were different from their northern neighbours is not clear. We know for exemple of silver necklaces bearing Pictish symbols at Traprain Law in Gododdin.
From a roman point of view, and only really from the 4th century onward (first mention of the Picts in AD 297, before we have several tribes including Caledonians, sometimes labelled Britons by Rome) it may just have been that Britons were part of the empire, or allied to it, and Picts were the hostile barbarians living north of the Firth of Forth. It's a bit reminding of Caesar's dichotomy between Celts and Germans, with Celts living on one side of the Rhine and Germans on the other whereas it was much more complex culturally speaking.

Picts likely spoke a P-celtic language, like Brittonic was. The arguments that they spoke a Q-celtic language or a non-celtic language seem to be ruled out by scholars as far I know. They were called Cruithni in gaelic, which I believe (but I may be wrong) is rendered Pritani in their own language, which is the same that... Britanni... So Picts may just have thought themselves as Britons!

There was no 'scottic' invasion or settlement by AD 600. Western Scotland was heavily influenced by Ireland since the Bronze Age, and gaelic-speaking population was probably living there centuries before the era that interest us. The 'Adventus scottorum' and the legendary foundation of scottish Dal Riada circa 500 AD may just have been a political shift of one gaelic dynasty to another.