By now, I have read enough of Clarke's stories to know that ETs happen to be one of his favorite themes. He tends to be quite graphical about their description - so we can reasonably enumerate his favorite designs.
1. Pure energy beings. Going by the characteristics described, these are indistinguishable from Hindu concept of brahm (also called many other names in India in various sects & regions). The meeting of such ETs with humans tends to culminate in humans merger into this higher being through a process that is essentially the Hindu concept of moksha.
It is for this reason that I tag these stories as religious.
In contrast, I have read of at least 2 cases of energy beings in Asimov's stories, & they tend to be essentially secular:
- A short story where 2 energy beings meet & recollect their conscience to discover that they used to be a human male & a human female, respectively, eons back in time.
- In one or two of the 6-7 sequels to Foundation Trilogy, there are energy beings wandering around the galaxy angry at humans for having driven them from their ancient homes during human colonization that created Galactic Empire.
3. In Childhood's End, apart from energy beings that appear towards the end, there are what the humans call Overlords - bird like beings that look like the Christian traditional pictures of devil.
4. In The Songs of Distant Earth, there are large crab-like sea creatures under the seas of the non-earth watery world where action occurs. These are as smart as chimpanzees, or on the verge of being as smart - I do not recall the exact details. This story also describes some under sea & overland vegetation of alien origin.
5. In Rama II, their is a brief appearance towards the end of an eel-like alien that lives in liquid seas. It talks to human heroin via robotic interpreters. If I recall correctly, it talks via a kind of what appears to human observer as dance; I don't recall well this specific mode of alien talk.
6. In the three Rama sequels, there are octopus like creatures that
- can live in both water & on land.
- make a kind of metallic sound when moving on land.
- are expert at genetic engineering, & themselves come in at least 3 different sizes: normal, extra large capable of storing food for community, & microscopic for interactions with bacteria-like genetic machines used for such things as surgery.
- talk using a band of colors, some of them outside the spectrum visible to unaided human eye.
- keep a human couple & their child in their zoo - the way humans keep zoo animals.
- Sponge- or cotton-like that are essentially pure mind, & are physically defenseless.
- These sponges produce eggs that humans call "mama melons".
- From "mama melons" hatch short-lived quick-moving somewhat cat-like & smart animals that do most interactions with sponges. They die by going inside sponge, which absorbs their tissue.
- Big birdlike animals capable of flight, but not very smart. They eat "mama melons". In return, they provide physical security to sponges & their quick-moving progeny; hence synergistic with them.