5th dnd – What constitutes an attack for the attacking action of a companion of the Beastmaster?

The player manual version of the Beast Master is quite explicit about what their mate can or can not do:

The beast obeys your orders as best as it can. It is your turn to your initiative. In turn, you can verbally command the beast to move (no action required on your part). You can use your action to verbally instruct him to perform Attack, Dash, Disengage, or Help actions.. If you do not give a command, the beast takes the Dodge action. Once you have the Extra Attack feature, you can make sure that a weapon attacks yourself when you order the Beast to take Attack action.

So, it will take Dodge unless you order him to take one of the following actions: Attack, Dash, Disengage and Help.

Swallow is not one of these actions. Although this involves the use of an attack, it is the same as spells such as Boomante blade or Green flame blade are not attack actions (which would make them otherwise usable with Additional attack or other similar features) but involve making melee attacks as part of their action.

Cloud of ink is in a similar situation. This is not an attack, so your companion can not use it.

The pet loses his multiattack action, if he has one.

The companion obeys your orders the best he can. He launches the initiative as any other creature, but you determine his actions, his decisions, his attitudes, and so on. If you are unable or absent, your companion acts alone.

These are very different restrictions than the Beast Master Ranger is allowed. For this version of the class, no restrictions are imposed on the actions that the mate can undertake, except that he may not use his multi-attack feature if equipped with it. So, if the companion has an action like Swallow or Cloud of inkhe would be perfectly able to use these features.

There is however a caveat: as a Conclave Beast Ranger, you are limited by the number of animals you can have:

With 8 hours of work and the expense of rare herbs and delicacies worth 50 in., You summon a desert animal to serve as a faithful companion. You normally choose your companion from the following animals: a monkey, a black bear, a wild boar, a giant badger, a giant weasel, a mule, a panther or a wolf. However, your DM can choose one of these animals for you, depending on the surrounding terrain and types of creatures that would be logically present in the area.

The language is permissive, and many DMs would probably allow other pets, much like the normal Beast Master Ranger, depending on your position and how you try to summon them. But by default, you would not have access to a giant frog or octopus, and none of the creatures listed by the entry have unusual abilities.